Use StrictConcurrency=complete in Package.swift and add Swift 6 language mode CI (#44)

### Motivation

We're trying to lock in strict concurrency across the package ecosystem
and standardise on how that's tested.

### Modifications

- Add `StrictConcurrency=complete` feature flag to all targets in the
package manifest
- Remove now superfluous strict concurrency compiler flag in CI runs
- Add a new CI pipeline that tests the package with Swift 6 language
mode.

### Result

- Package configuration consistent with others adopting strict
concurrency.
- Swift 6 language mode pipeline established to catch regressions.

Author: simonjbeaumont

Diff for: .github/workflows/pull_request.yml

@@ -15,13 +15,16 @@ jobs:
     name: Unit tests
     uses: apple/swift-nio/.github/workflows/unit_tests.yml@main
     with:
-      linux_5_9_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -strict-concurrency=complete"
-      # TODO: `enable -Xswiftc -strict-concurrency=complete`
+      linux_5_9_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
       linux_5_10_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
-      linux_6_0_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable -Xswiftc -strict-concurrency=complete"
+      linux_6_0_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
       linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
       linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
 
   cxx-interop:
     name: Cxx interop
     uses: apple/swift-nio/.github/workflows/cxx_interop.yml@main
+
+  swift-6-language-mode:
+    name: Cxx interop
+    uses: apple/swift-nio/.github/workflows/swift_6_language_mode.yml@main

Diff for: Package.swift

@@ -19,7 +19,8 @@ import PackageDescription
 let swiftSettings: [SwiftSetting] = [
     // https://github.com/apple/swift-evolution/blob/main/proposals/0335-existential-any.md
     // Require `any` for existential types.
-    .enableUpcomingFeature("ExistentialAny")
+    .enableUpcomingFeature("ExistentialAny"),
+    .enableExperimentalFeature("StrictConcurrency=complete"),
 ]
 
 let package = Package(