Bump actions/checkout from 5 to 6 in the github-actions group (#41)

Bumps the github-actions group with 1 update: [actions/checkout](https://github.com/actions/checkout).


Updates `actions/checkout` from 5 to 6
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
  dependency-group: github-actions
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/upstream-sync.yml

@@ -33,7 +33,7 @@ jobs:
     steps:
       # Step 1: Checkout target repo
       - name: Checkout target repo
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           ref: tools
           # Required for pushing changes