Update tibdex/github-app-token action to v2 (#31)

Signed-off-by: mend-for-github-com[bot] <mend-for-github-com[bot]@users.noreply.github.com>
Co-authored-by: mend-for-github-com[bot] <50673670+mend-for-github-com[bot]@users.noreply.github.com>
(cherry picked from commit 9655f07)
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

.github/workflows/backport.yml

@@ -26,7 +26,7 @@ jobs:
     steps:
       - name: GitHub App token
         id: github_app_token
-        uses: tibdex/github-app-token@v1
+        uses: tibdex/github-app-token@v2
         with:
           app_id: ${{ secrets.APP_ID }}
           private_key: ${{ secrets.APP_PRIVATE_KEY }}