Bump actions/checkout from 4 to 6 (#5015)

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

- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c

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

.github/workflows/labled_doctest.yaml

@@ -66,7 +66,7 @@ jobs:
           git --no-pager log -1 || true
 
       - name: Checkout vllm-project/vllm-ascend repo
-        uses: actions/checkout@v6.0.1
+        uses: actions/checkout@v6
 
       - name: Run vllm-ascend/tests/e2e/run_doctests.sh
         run: |