ci: dedicated token for Lango stale PRs

Default token for actions provided by GitHub lacks
read and write permissions for private projects. This
commit changes the token used for the stale PRs workflow
to the dedicated token with needed permissions.

NO_DOC=Workflow fix
NO_TEST=Workflow fix
NO_CHANGELOG=Workflow fix

.github/workflows/lango-stale-reviews.yml

@@ -2,11 +2,12 @@ name: lango-stale-reviews
 on:
   schedule:
     - cron:  '* */1 * * *'    # At every hour.
+  workflow_dispatch:
 
 jobs:
   update-last-activity-dates:
     env:
-      LANGO_REVIEW_BOARD_TOKEN: ${{ github.token }}
+      LANGO_REVIEW_BOARD_TOKEN: ${{ secrets.LANGO_REVIEW_BOARD_TOKEN }}
       ORGANIZATION: Tarantool
       PROJECT_ID: 83
     runs-on: ubuntu-latest