Raise minimum swift version to 5.8 (#147)

Motivation:

It's consistant with support policy (see https://github.com/apple/swift-metrics/pull/141).
It matches what's being tested.
It will make future changes adopting new features easier.

Modifications:

Bump swift tools version to 5.8
Remove linux tests and test discovery flag which are not required.

Result:

Will not longer build using swift before 5.8 - older compilers will find older
versions of this library when resolving versions.

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.6
+// swift-tools-version:5.8
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the Swift Metrics API open source project

Tests/LinuxMain.swift

@@ -1,32 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// This source file is part of the Swift Metrics API open source project
-//
-// Copyright (c) 2018-2019 Apple Inc. and the Swift Metrics API project authors
-// Licensed under Apache License v2.0
-//
-// See LICENSE.txt for license information
-// See CONTRIBUTORS.txt for the list of Swift Metrics API project authors
-//
-// SPDX-License-Identifier: Apache-2.0
-//
-//===----------------------------------------------------------------------===//
-//
-// LinuxMain.swift
-//
-import XCTest
-
-///
-/// NOTE: This file was generated by generate_linux_tests.rb
-///
-/// Do NOT edit this file directly as it will be regenerated automatically when needed.
-///
-
-#if os(Linux) || os(FreeBSD)
-@testable import MetricsTests
-
-XCTMain([
-    testCase(MetricsExtensionsTests.allTests),
-    testCase(MetricsTests.allTests),
-])
-#endif

docker/docker-compose.yaml

@@ -28,7 +28,7 @@ services:
 
   test:
     <<: *common
-    command: /bin/bash -xcl "swift test -Xswiftc -warnings-as-errors --enable-test-discovery $${SANITIZER_ARG-}"
+    command: /bin/bash -xcl "swift test -Xswiftc -warnings-as-errors $${SANITIZER_ARG-}"
 
   # util