Bump actions/checkout from 4 to 6 (#4380)

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.

- vLLM main:
2918c1b49c

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

.github/workflows/image_a3_ubuntu.yml

@@ -58,7 +58,7 @@ jobs:
     runs-on: ubuntu-latest
     if: ${{ github.event_name == 'push' || (contains(github.event.pull_request.labels.*.name, 'ready') && contains(github.event.pull_request.labels.*.name, 'ready-for-test')) }}
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
       with:
         fetch-depth: 0
         persist-credentials: false