chore(ci): update checkout to v6 and add dependabot

Update actions/checkout to the latest version (v6) for improved
performance and security. Add Dependabot configuration to automatically
check for and create PRs for GitHub Actions dependency updates weekly.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: DennisOSRM

.github/dependabot.yml

@@ -0,0 +1,9 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    commit-message:
+      prefix: "chore(deps)"
+      include: "scope"

.github/workflows/sanity-checks.yml

@@ -10,7 +10,7 @@ jobs:
   sanity-checks:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Check JSON validity
         run: |