Drop Swift 5.10, Use Swift 6 language mode (#525)

Co-authored-by: Rick Newton-Rogers <rnro@apple.com>

Author: fabianfett

.github/workflows/main.yml

@@ -14,7 +14,6 @@ jobs:
     name: Unit tests
     uses: apple/swift-nio/.github/workflows/unit_tests.yml@main
     with:
-      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"
       linux_6_1_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
       linux_6_2_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"

.github/workflows/pull_request.yml

@@ -18,7 +18,6 @@ jobs:
     name: Unit tests
     uses: apple/swift-nio/.github/workflows/unit_tests.yml@main
     with:
-      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"
       linux_6_1_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
       linux_6_2_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"

IntegrationTests/tests_01_allocation_counters/Thresholds/5.10.json

@@ -1,4 +1,4 @@
-// swift-tools-version:5.10
+// swift-tools-version:6.0
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the SwiftNIO open source project
@@ -19,10 +19,6 @@ let strictConcurrencyDevelopment = false
 
 let strictConcurrencySettings: [SwiftSetting] = {
     var initialSettings: [SwiftSetting] = []
-    initialSettings.append(contentsOf: [
-        .enableUpcomingFeature("StrictConcurrency"),
-        .enableUpcomingFeature("InferSendableFromCaptures"),
-    ])
 
     if strictConcurrencyDevelopment {
         // -warnings-as-errors here is a workaround so that IDE-based development can

Package.swift

@@ -35,7 +35,8 @@ SwiftNIO HTTP/2     | Minimum Swift Version
 `1.27.0 ..< 1.29.0` | 5.6
 `1.29.0 ..< 1.31.0` | 5.7
 `1.31.0 ..< 1.37.0` | 5.8
-`1.37.0 ...`        | 5.9
+`1.37.0 ..< 1.39.0` | 5.9
+`1.39.0 ...` | 6.0
 
 ### `swift-nio-http2` 0.x