Trim upstream-specific workflows + CODEOWNERS

Drop workflows we don't need on the fork:
- licensed.yml (no .licenses for @google-cloud/storage)
- workflow.yml (jest tests mock @actions/cache, break with wrapper)
- close-inactive-issues, issue-opened, pr-opened (upstream oncall)
- publish-immutable-actions (not publishing to marketplace)
- release-new-action-version (replaced by auto-tag.yml)

Drop CODEOWNERS so merges don't require @actions/actions-cache review.
Drop .licensed.yml config (no longer used).

.github/workflows/pr-opened-workflow.yml

@@ -1,25 +0,0 @@
-name: Assign pull request reviewer
-
-on:
-  pull_request_target:
-    types: [opened]
-
-permissions:
-  pull-requests: write
-
-jobs:
-  run-action:
-    runs-on: ubuntu-latest
-    steps:
-      - name: Get current oncall
-        id: oncall
-        run: |
-          echo "CURRENT=$(curl --request GET 'https://api.pagerduty.com/oncalls?include[]=users&schedule_ids[]=P5VG2BX&earliest=true' --header 'Authorization: Token token=${{ secrets.PAGERDUTY_TOKEN }}' --header 'Accept: application/vnd.pagerduty+json;version=2' --header 'Content-Type: application/json' | jq -r '.oncalls[].user.name')" >> $GITHUB_OUTPUT
-
-      - name: Request Review
-        run: |
-          curl -X POST -H "Accept: application/vnd.github+json" -H "Authorization: Bearer ${{ secrets.GITHUB_TOKEN}}" https://api.github.com/repos/${{github.repository}}/pulls/${{ github.event.pull_request.number}}/requested_reviewers -d '{"reviewers":["${{steps.oncall.outputs.CURRENT}}"]}'
-
-      - name: Add Assignee
-        run: |
-          curl -X POST -H "Accept: application/vnd.github+json" -H "Authorization: Bearer ${{ secrets.GITHUB_TOKEN}}" https://api.github.com/repos/${{github.repository}}/issues/${{ github.event.pull_request.number}}/assignees -d '{"assignees":["${{steps.oncall.outputs.CURRENT}}"]}'