Drop Swift 5.9 (#66)

Motivation:

Swift 5.9 is no longer supported, we should bump the tools version and
remove it from our CI.

Modifications:

* Bump the Swift tools version to Swift 5.10
* Remove Swift 5.9 jobs where appropriate in main.yml, pull_request.yml

Result:

Code reflects our support window.

Author: rnro

.github/workflows/main.yml

@@ -11,7 +11,6 @@ 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"
             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"
             linux_6_1_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"

.github/workflows/pull_request.yml

@@ -14,7 +14,6 @@ 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"
             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"
             linux_6_1_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.8
+// swift-tools-version:5.10
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the SwiftNIO open source project