Bump to Swift 6.0

5.9 has issues with swift-markdown's use of versioned `Package.swift`
after the added use of unsafe flags (which are 6.2 only). 6.0 is over a
year old now.

Author: bnbarham

.github/workflows/pull_request.yml

@@ -20,7 +20,7 @@ jobs:
     uses: swiftlang/github-workflows/.github/workflows/swift_package_test.yml@main
     with:
       enable_cross_pr_testing: true
-      # "5.9" is excluded because of https://github.com/swiftlang/swift-format/issues/1094.
+      linux_exclude_swift_versions: "[{\"swift_version\": \"5.9\"}, {\"swift_version\": \"5.10\"}]"
       windows_exclude_swift_versions: "[{\"swift_version\": \"5.9\"}]"
   soundness:
     name: Soundness

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 5.9
+// swift-tools-version: 6.0
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the Swift.org open source project