Drop Swift 5.9 (#515)

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.

---------

Co-authored-by: George Barnett <[email protected]>

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: "--explicit-target-dependency-import-check error"
       linux_5_10_arguments_override: "--explicit-target-dependency-import-check error"
       linux_6_0_arguments_override: "--explicit-target-dependency-import-check error"
       linux_6_1_arguments_override: "--explicit-target-dependency-import-check error"

.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: "--explicit-target-dependency-import-check error"
       linux_5_10_arguments_override: "--explicit-target-dependency-import-check error"
       linux_6_0_arguments_override: "--explicit-target-dependency-import-check error"
       linux_6_1_arguments_override: "--explicit-target-dependency-import-check error"

IntegrationTests/tests_01_allocation_counters/Thresholds/5.9.json

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

Package.swift

@@ -24,7 +24,7 @@ To depend on `swift-nio-http2`, put the following in the `dependencies` of your
 
     .package(url: "https://github.com/apple/swift-nio-http2.git", from: "1.19.2"),
 
-The most recent versions of SwiftNIO HTTP/2 support Swift 5.7 and newer. The minimum Swift version supported for SwiftNIO HTTP/2 releases are detailed below:
+The minimum Swift version supported for SwiftNIO HTTP/2 releases are detailed below:
 
 SwiftNIO HTTP/2     | Minimum Swift Version
 --------------------|----------------------
@@ -34,8 +34,8 @@ SwiftNIO HTTP/2     | Minimum Swift Version
 `1.24.0 ..< 1.27.0` | 5.5.2
 `1.27.0 ..< 1.29.0` | 5.6
 `1.29.0 ..< 1.31.0` | 5.7
-`1.31.0 ...`        | 5.8
-
+`1.31.0 ..< 1.37.0` | 5.8
+`1.37.0 ...`        | 5.9
 
 ### `swift-nio-http2` 0.x