Trying GitHub actions with Swift 5.10, since fails with Swift 6.

.github/workflows/documentation.yml

@@ -28,7 +28,7 @@ jobs:
       - name: Setup Swift
         uses: swift-actions/setup-swift@v1
         with:
-          swift-version: "6.0"
+          swift-version: "5.10"
       - name: Build documentation
         run: |
           swift package --allow-writing-to-directory ./docs \