Increase swift tools version to 5.9

We no longer need to support Swift 5.8

Author: ahoppen

.github/workflows/pull_request.yml

@@ -8,6 +8,8 @@ jobs:
   tests:
     name: Test
     uses: swiftlang/github-workflows/.github/workflows/swift_package_test.yml@main
+    with: 
+      linux_exclude_swift_versions: "[{\"swift_version\": \"5.8\"}]"
   soundness:
     name: Soundness
     uses: swiftlang/github-workflows/.github/workflows/soundness.yml@main

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.8
+// swift-tools-version: 5.9
 
 import Foundation
 import PackageDescription