GHA: Attempt fix exceeded a secondary rate limit

Frequent but intermittently, the stale issue and PR locking workflow
generates the error:

```
You have exceeded a secondary rate limit. Please wait a few minutes
before you try again. If you reach out to GitHub Support for help,
please include the request ID XYZ
```

According to upstream `dessant/lock-threads` issue 48, this seems to be
coming from the GitHub side (bug/feature/limitation), since the action
uses an official github API rate-limiting library.  It's unlikely related
to which style/syntax of github token is used, nor if the action is
executed concurrently across multiple repos.

According to the rate-limiting docs:
https://docs.github.com/en/rest/using-the-rest-api/rate-limits-for-the-rest-api?apiVersion=2022-11-28#about-secondary-rate-limits
it's possible the issue is caused due to an unknown aspect of the clause:

```
These secondary rate limits are subject to change without notice. You
may also encounter a secondary rate limit for undisclosed reasons.
```

The same docs indicate Github Apps have enhanced rate-limits which
scale with the org's repo count.  Attempt to fix the intermittent
failures by making use of a new, dedicated, org-specific, private "Stale
Locking App" I recently created.  This requires the addition of a new
action to the workflow that obtains a short-lived token for passing to
lock-threads.

Note: Because both `vars.STALE_LOCKING_APP_ID` and
`secrets.STALE_LOCKING_APP_PRIVATE_KEY` are defined at the
containers-organization level, the Buildah and Skopeo re-use
of this workflow should continue to function normally w/o change.

Signed-off-by: Chris Evich <cevich@redhat.com>

.github/workflows/issue_pr_lock.yml

@@ -15,6 +15,8 @@ on:
   # Ref: https://docs.github.com/en/actions/using-workflows/reusing-workflows
   workflow_call:
     secrets:
+      STALE_LOCKING_APP_PRIVATE_KEY:
+        required: true
       ACTION_MAIL_SERVER:
         required: true
       ACTION_MAIL_USERNAME:
@@ -48,9 +50,20 @@ jobs:
       issues: write
       pull-requests: write
     steps:
+      # Use dedicated github app to workaround API rate limiting
+      # Ref: https://docs.github.com/en/apps/creating-github-apps/authenticating-with-a-github-app/making-authenticated-api-requests-with-a-github-app-in-a-github-actions-workflow
+      - name: Obtain Stale Locking App token
+        id: generate-token
+        uses: actions/create-github-app-token@v1
+        with:
+          # N/B: These are both defined at the containers-org level
+          app-id: ${{ vars.STALE_LOCKING_APP_ID }}
+          private-key: ${{ secrets.STALE_LOCKING_APP_PRIVATE_KEY }}
+
       # Ref: https://github.com/dessant/lock-threads#usage
       - uses: dessant/lock-threads@v5
         with:
+          github-token: '${{ steps.generate-token.outputs.token }}'
           process-only: 'issues, prs'
           issue-inactive-days: '${{env.CLOSED_DAYS}}'
           pr-inactive-days: '${{env.CLOSED_DAYS}}'