Update actions/cache from v5 to v6

Author: greboid

.github/workflows/update-containers.yml.gotpl

@@ -28,7 +28,7 @@ jobs:
           id: get-sha
           run: echo "sha=$(git ls-remote --heads https://github.com/greboid/contempt.git splitupdating | cut -f1)" >> "$GITHUB_OUTPUT"
         - name: Cache contempt
-          uses: actions/cache@v5
+          uses: actions/cache@v6
           id: cache
           with:
               path: ~/go/bin/contempt