Bump actions/checkout from 4 to 6 (#38464)

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...v6)

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

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

Author: dependabot[bot]

.github/workflows/beam_PostCommit_Java_ValidatesRunner_Spark4StructuredStreaming.yml

@@ -63,7 +63,7 @@ jobs:
       (github.event_name == 'schedule' && github.repository == 'apache/beam') ||
       github.event.comment.body == 'Run Spark4 StructuredStreaming ValidatesRunner'
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Setup repository
         uses: ./.github/actions/setup-action
         with: