GHA: Use 5.9

NIO dropped support for 5.8.

Author: helje5

.github/workflows/swift.yml

@@ -13,7 +13,7 @@ jobs:
       fail-fast: false
       matrix:
         image:
-          - swift:5.8-jammy
+          - swift:5.9-jammy
           - swift:6.1-noble
     container: ${{ matrix.image }}
     steps: