chore(deps): bump google-github-actions/auth in /.github/workflows

Bumps [google-github-actions/auth](https://github.com/google-github-actions/auth) from 2.1.7 to 2.1.8.
- [Release notes](https://github.com/google-github-actions/auth/releases)
- [Changelog](https://github.com/google-github-actions/auth/blob/main/CHANGELOG.md)
- [Commits](google-github-actions/auth@6fc4af4...71f9864)

---
updated-dependencies:
- dependency-name: google-github-actions/auth
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

.github/workflows/prepare-pr.yml

@@ -24,7 +24,7 @@ jobs:
     steps:      
       - name: Auth gcloud
         id: gauth
-        uses: google-github-actions/auth@6fc4af4b145ae7821d527454aa9bd537d1f2dc5f # v2.1.7
+        uses: google-github-actions/auth@71f986410dfbc7added4569d411d040a91dc6935 # v2.1.8
         with:
           workload_identity_provider: '${{ secrets.GWIP }}'
           service_account: '${{ secrets.GSA }}'
@@ -72,7 +72,7 @@ jobs:
     steps:
       - name: Auth gcloud
         id: gauth
-        uses: google-github-actions/auth@6fc4af4b145ae7821d527454aa9bd537d1f2dc5f # v2.1.7
+        uses: google-github-actions/auth@71f986410dfbc7added4569d411d040a91dc6935 # v2.1.8
         with:
           workload_identity_provider: '${{ secrets.GWIP }}'
           service_account: '${{ secrets.GSA }}'
@@ -118,7 +118,7 @@ jobs:
     steps:
       - name: Auth gcloud
         id: gauth
-        uses: google-github-actions/auth@6fc4af4b145ae7821d527454aa9bd537d1f2dc5f # v2.1.7
+        uses: google-github-actions/auth@71f986410dfbc7added4569d411d040a91dc6935 # v2.1.8
         with:
           workload_identity_provider: '${{ secrets.GWIP }}'
           service_account: '${{ secrets.GSA }}'
@@ -161,7 +161,7 @@ jobs:
     steps:
       - name: Auth gcloud
         id: gauth
-        uses: google-github-actions/auth@6fc4af4b145ae7821d527454aa9bd537d1f2dc5f # v2.1.7
+        uses: google-github-actions/auth@71f986410dfbc7added4569d411d040a91dc6935 # v2.1.8
         with:
           workload_identity_provider: '${{ secrets.GWIP }}'
           service_account: '${{ secrets.GSA }}'