Fix robot CLA (#6206)

Per [b/361612219](https://b.corp.google.com/issues/361612219),

This updates the committer email we use in our post release cleanup to
use the proper robot email, so that it properly passes CLA checks.

Author: daymxn

.github/workflows/post_release_cleanup.yml

@@ -29,7 +29,7 @@ jobs:
         uses: peter-evans/create-pull-request@v4
         with:
           token: ${{ secrets.GOOGLE_OSS_BOT_TOKEN }}
-          committer: google-oss-bot <google[email protected]>
+          committer: google-oss-bot <firebase[email protected]>
           assignees: ${{ github.actor }}
           base: 'main'
           branch: 'releases/${{ inputs.name }}.mergeback'