Bump minimum Swift version to 5.7 (#189)

Motivation:

Now that Swift 5.9 is GM we should update the supported versions and
remove 5.6

Modifications:

* Update `Package.swift`
* Delete the 5.6 docker compose file and make a 5.10 one
* Update docs

Result:

Remove support for Swift 5.6, add 5.10

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.6
+// swift-tools-version:5.7
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the SwiftNIO open source project

README.md

@@ -48,13 +48,13 @@ declaring [SwiftNIO's Public API](https://github.com/apple/swift-nio/blob/main/d
 
 ### `swift-nio-transport-services ` 1.x
 
-`swift-nio-transport-services` versions 1.x is part of the SwiftNIO 2 family of repositories and does not have any dependencies besides [`swift-nio`](https://github.com/apple/swift-nio), Swift 5.6, and an Apple OS supporting `Network.framework`. As the latest version, it lives on the [`main`](https://github.com/apple/swift-nio-transport-services) branch.
+`swift-nio-transport-services` versions 1.x is part of the SwiftNIO 2 family of repositories and does not have any dependencies besides [`swift-nio`](https://github.com/apple/swift-nio), Swift 5.7, and an Apple OS supporting `Network.framework`. As the latest version, it lives on the [`main`](https://github.com/apple/swift-nio-transport-services) branch.
 
 To depend on `swift-nio-transport-services `, put the following in the `dependencies` of your `Package.swift`:
 
     .package(url: "https://github.com/apple/swift-nio-transport-services.git", from: "1.0.0"),
 
-The most recent versions of SwiftNIO Transport Services support Swift 5.6 and newer. The minimum Swift version supported by SwiftNIO Transport Services releases are detailed below:
+The most recent versions of SwiftNIO Transport Services support Swift 5.7 and newer. The minimum Swift version supported by SwiftNIO Transport Services releases are detailed below:
 
 SwiftNIO Extras     | Minimum Swift Version
 --------------------|----------------------
@@ -62,7 +62,8 @@ SwiftNIO Extras     | Minimum Swift Version
 `1.11.0 ..< 1.12.0` | 5.2
 `1.12.0 ..< 1.15.0` | 5.4
 `1.15.0 ..< 1.17.0` | 5.5.2
-`1.17.0 ...`        | 5.6
+`1.17.0 ..< 1.19.0` | 5.6
+`1.19.0 ...`        | 5.7
 
 ### `swift-nio-transport-services ` 0.x
 

Sources/NIOTransportServices/Docs.docc/index.md

@@ -50,13 +50,13 @@ declaring [SwiftNIO's Public API](https://github.com/apple/swift-nio/blob/main/d
 
 #### NIO Transport Services 1.x
 
-`swift-nio-transport-services` versions 1.x is part of the SwiftNIO 2 family of repositories and does not have any dependencies besides [`swift-nio`](https://github.com/apple/swift-nio), Swift 5.6, and an Apple OS supporting `Network.framework`. As the latest version, it lives on the [`main`](https://github.com/apple/swift-nio-transport-services) branch.
+`swift-nio-transport-services` versions 1.x is part of the SwiftNIO 2 family of repositories and does not have any dependencies besides [`swift-nio`](https://github.com/apple/swift-nio), Swift 5.7, and an Apple OS supporting `Network.framework`. As the latest version, it lives on the [`main`](https://github.com/apple/swift-nio-transport-services) branch.
 
 To depend on `swift-nio-transport-services `, put the following in the `dependencies` of your `Package.swift`:
 
     .package(url: "https://github.com/apple/swift-nio-transport-services.git", from: "1.0.0"),
 
-The most recent versions of SwiftNIO Transport Services support Swift 5.6 and newer. The minimum Swift version supported by SwiftNIO Transport Services releases are detailed below:
+The most recent versions of SwiftNIO Transport Services support Swift 5.7 and newer. The minimum Swift version supported by SwiftNIO Transport Services releases are detailed below:
 
 SwiftNIO Extras     | Minimum Swift Version
 --------------------|----------------------
@@ -64,7 +64,8 @@ SwiftNIO Extras     | Minimum Swift Version
 `1.11.0 ..< 1.12.0` | 5.2
 `1.12.0 ..< 1.14.0` | 5.4
 `1.15.0 ..< 1.17.0` | 5.5.2
-`1.17.0 ...`        | 5.6
+`1.17.0 ..< 1.19.0` | 5.6
+`1.19.0 ...`        | 5.7
 
 #### NIO Transport Services 0.x
 

docker/docker-compose.2004.56.yaml

@@ -1,19 +0,0 @@
-version: "3"
-
-services:
-
-  runtime-setup:
-    image: swift-nio-transport-services:20.04-5.6
-    build:
-      args:
-        ubuntu_version: "focal"
-        swift_version: "5.6"
-
-  documentation-check:
-    image: swift-nio-transport-services:20.04-5.6
-
-  test:
-    image: swift-nio-transport-services:20.04-5.6
-
-  shell:
-    image: swift-nio-transport-services:20.04-5.6

docker/docker-compose.2204.510.yaml

@@ -0,0 +1,19 @@
+version: "3"
+
+services:
+
+  runtime-setup:
+    image: swift-nio-transport-services:22.04-5.10
+    build:
+      args:
+        ubuntu_version: "jammy"
+        base_image: "swiftlang/swift:nightly-5.10-jammy"
+
+  documentation-check:
+    image: swift-nio-transport-services:22.04-5.10
+
+  test:
+    image: swift-nio-transport-services:22.04-5.10
+
+  shell:
+    image: swift-nio-transport-services:22.04-5.10

docker/docker-compose.2204.59.yaml

@@ -7,7 +7,7 @@ services:
     build:
       args:
         ubuntu_version: "jammy"
-        base_image: "swiftlang/swift:nightly-5.9-jammy"
+        swift_version: "5.9"
 
   documentation-check:
     image: swift-nio-transport-services:22.04-5.9