Raise minimum swift version to 5.8 (#147)

Motivation:

It's consistant with support policy (see #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.

Author: PeterAdams-A

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

@@ -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