GHA: Build against Swift 6

... and drop those which do not work on
GHA anymore.
Also uses checkout v4 and latest Xcode.

.github/workflows/swift.yml

@@ -13,13 +13,13 @@ jobs:
       fail-fast: false
       matrix:
         image:
-          - swift:5.5.3-xenial
-          - swift:5.6.1-bionic
-          - swift:5.7.2-focal
+          - swift:5.9.2-focal
+          - swift:5.10-jammy
+          - swift:6.0-noble
     container: ${{ matrix.image }}
     steps:
       - name: Checkout Repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Build Swift Debug Package
         run: swift build -c debug
       - name: Build Swift Release Package
@@ -30,9 +30,9 @@ jobs:
       - name: Select latest available Xcode
         uses: maxim-lobanov/[email protected]
         with:
-          xcode-version: 13.2.1
+          xcode-version: latest
       - name: Checkout Repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Build Swift Debug Package
         run: swift build -c debug
       - name: Build Swift Release Package