Update actions/checkout action to v7

Author: renovate[bot]

.github/workflows/tailscale.yaml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v7
 
       - name: Test ACL
         if: github.event_name == 'pull_request'