Update backport workflow to use default GITHUB_TOKEN

Use the default token injected into workflows since the GH_BOT_ACCESS_TOKEN is revoked.

The default token doesn't have the permission used for unassigning default reviewers. If you don't have it, you get cryptic comments posted back on the PR after the backport PRs are opened with the message `Validation Failed: "Could not resolve to a node with the global id of '<NODE ID>."`

Like in grafana/pyroscope#3557 (comment)

Behavior was documented in grafana/grafana-github-actions#226 and made configurable in 
grafana/grafana-github-actions#227

`removeDefaultReviewers: false` instructs the workflow to not try to avoid the error but will of course not remove the default reviewers assigned on the backport PR.

.github/workflows/backport.yml

@@ -14,14 +14,13 @@ jobs:
         with:
           repository: "grafana/grafana-github-actions"
           path: ./actions
-          # Pin the version to before https://github.com/grafana/grafana-github-actions/pull/113 because
-          # to avoid the strict rules for PR labels.
-          ref: d284afd314ca3625c23595e9f62b52d215ead7ce
       - name: Install Actions
         run: npm install --production --prefix ./actions
       - name: Run backport
         uses: ./actions/backport
         with:
-          token: ${{secrets.GH_BOT_ACCESS_TOKEN}}
-          labelsToAdd: "backport"
+          labelsToAdd: backport
+          # The provided token needs read permissions for organization members if you want to remove the default reviewers.
+          removeDefaultReviewers: false
           title: "[{{base}}] {{originalTitle}}"
+          token: ${{ secrets.GITHUB_TOKEN }}