chore: Update actions/checkout to v6 in GitHub workflows

V4 still uses deprecated Node.js 20.

Co-authored-by: Liang Wu <wuliang@google.com>
PiperOrigin-RevId: 882275971

Author: wuliang229

.github/workflows/release-cherry-pick.yml

@@ -18,7 +18,7 @@ jobs:
   cherry-pick:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           ref: release/candidate
           fetch-depth: 0

.github/workflows/release-cut.yml

@@ -18,7 +18,7 @@ jobs:
   cut-release:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           ref: ${{ inputs.commit_sha || 'main' }}
 

.github/workflows/release-finalize.yml

@@ -29,7 +29,7 @@ jobs:
             echo "is_release_pr=false" >> $GITHUB_OUTPUT
           fi
 
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         if: steps.check.outputs.is_release_pr == 'true'
         with:
           ref: release/candidate

.github/workflows/release-please.yml

@@ -27,7 +27,7 @@ jobs:
             echo "exists=false" >> $GITHUB_OUTPUT
           fi
 
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         if: steps.check.outputs.exists == 'true'
         with:
           ref: release/candidate

.github/workflows/release-publish.yml

@@ -28,7 +28,7 @@ jobs:
           echo "version=$VERSION" >> $GITHUB_OUTPUT
           echo "Publishing version: $VERSION"
 
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Install uv
         uses: astral-sh/setup-uv@v4