Drop Swift 5.9 (#74)

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 -Xswiftc -strict-concurrency=complete --explicit-target-dependency-import-check error"
       # Disable strict concurrency checking as it intersects badly with
       # warnings-as-errors on 5.10 and later as SwiftPMs generated test manifest
       # has a non-sendable global property.

.github/workflows/pull_request.yml

@@ -15,7 +15,6 @@ jobs:
     name: Unit tests
     uses: apple/swift-nio/.github/workflows/unit_tests.yml@main
     with:
-      linux_5_9_arguments_override: "-Xswiftc -warnings-as-errors -Xswiftc -strict-concurrency=complete --explicit-target-dependency-import-check error"
       # Disable strict concurrency checking as it intersects badly with
       # warnings-as-errors on 5.10 and later as SwiftPMs generated test manifest
       # has a non-sendable global property.

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 5.9
+// swift-tools-version:5.10
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the SwiftOpenAPIGenerator open source project