Update actions/checkout action to v7

Author: renovate[bot]

.github/workflows/tailscale.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v7
 
       - name: Validate HuJSON
         run: python3 tailscale/validate-acl.py tailscale/acl.hujson