Update macOS CI to Swift 6

Author: mickeyl

.github/workflows/swift.yml

@@ -7,8 +7,11 @@ jobs:
     name: Swift ${{ matrix.swift }} on ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest]
-        swift: ["5.10"]
+        include:
+          - os: ubuntu-latest
+            swift: "5.10"
+          - os: macos-latest
+            swift: "6.0"
     runs-on: ${{ matrix.os }}
     steps:
       - uses: swift-actions/setup-swift@v2