Drop support for Swift 5.2 and 5.3 (#141)

As outlined in a [Swift forums post in November ’21](https://forums.swift.org/t/swiftnio-swift-version-support/53232), SwiftNIO will only support the latest non-patch Swift release and the 2 immediately prior non-patch versions.

- drop support for Swift 5.2 and 5.3. 
- update CI for Swift 5.4 to run on bionic instead of focal to ensure that we still test bionic. 
- add a CI job for Swift 5.7

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.2
+// swift-tools-version:5.4
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the SwiftNIO open source project

README.md

@@ -63,12 +63,14 @@ 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, 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 5.4, 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"),
 
+`swift-nio-transport-services` version 1.11.x and earlier support Swift 5.2 and 5.3. Earlier 1.x versions also support Swift 5.0 and 5.1.
+
 ### `swift-nio-transport-services ` 0.x
 
 The legacy `swift-nio-transport-services` 0.x is part of the SwiftNIO 1 family of repositories and works with Swift 4.1 and newer. The source code can be found on the [`swift-nio-transport-services-swift-4-maintenance`](https://github.com/apple/swift-nio-transport-services/tree/swift-nio-transport-services-swift-4-maintenance) branch.

docker/docker-compose.1604.52.yaml

@@ -1,16 +0,0 @@
-version: "3"
-
-services:
-
-  runtime-setup:
-    image: swift-nio-transport-services:16.04-5.2
-    build:
-      args:
-        ubuntu_version: "xenial"
-        swift_version: "5.2"
-
-  test:
-    image: swift-nio-transport-services:16.04-5.2
-
-  shell:
-    image: swift-nio-transport-services:16.04-5.2

docker/docker-compose.1804.53.yaml

@@ -1,16 +0,0 @@
-version: "3"
-
-services:
-
-  runtime-setup:
-    image: swift-nio-transport-services:18.04-5.3
-    build:
-      args:
-        ubuntu_version: "bionic"
-        swift_version: "5.3"
-
-  test:
-    image: swift-nio-transport-services:18.04-5.3
-
-  shell:
-    image: swift-nio-transport-services:18.04-5.3

docker/docker-compose.1804.54.yaml

@@ -0,0 +1,16 @@
+version: "3"
+
+services:
+
+  runtime-setup:
+    image: swift-nio-transport-services:18.04-5.4
+    build:
+      args:
+        ubuntu_version: "bionic"
+        swift_version: "5.4"
+
+  test:
+    image: swift-nio-transport-services:18.04-5.4
+
+  shell:
+    image: swift-nio-transport-services:18.04-5.4

docker/docker-compose.2004.54.yaml

@@ -1,16 +0,0 @@
-version: "3"
-
-services:
-
-  runtime-setup:
-    image: swift-nio-transport-services:20.04-5.4
-    build:
-      args:
-        ubuntu_version: "focal"
-        swift_version: "5.4"
-
-  test:
-    image: swift-nio-transport-services:20.04-5.4
-
-  shell:
-    image: swift-nio-transport-services:20.04-5.4

docker/docker-compose.2004.56.yaml

@@ -7,7 +7,7 @@ services:
     build:
       args:
         ubuntu_version: "focal"
-        base_image: "swiftlang/swift:nightly-5.6-focal"
+        swift_version: "5.6"
 
   test:
     image: swift-nio-transport-services:20.04-5.6

docker/docker-compose.2004.57.yaml

@@ -0,0 +1,16 @@
+version: "3"
+
+services:
+
+  runtime-setup:
+    image: swift-nio-transport-services:20.04-5.7
+    build:
+      args:
+        ubuntu_version: "focal"
+        base_image: "swiftlang/swift:nightly-main-focal"
+
+  test:
+    image: swift-nio-transport-services:20.04-5.7
+
+  shell:
+    image: swift-nio-transport-services:20.04-5.7