Bump actions/checkout from 3 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...v6)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/flat.yml

@@ -18,7 +18,7 @@ jobs:
         with:
           deno-version: v2.x
       - name: Check out repo
-        uses: actions/checkout@v3
+        uses: actions/checkout@v6
       - name: Fetch data (InterPlanetary One - Verifiers)
         uses: AlexxNica/flat@main
         with: