Upgrade actions/checkout to v6

Author: aaronfranke

.github/workflows/static_checks.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v6
 
       # Azure repositories are not reliable, we need to prevent Azure giving us packages.
       - name: Make apt sources.list use the default Ubuntu repositories