Update actions/checkout action from v2 to v4 (.github/workflows/slack-pr-reminder.yml) (#12146)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/slack-pr-reminder.yml

@@ -11,7 +11,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
         with:
           # Only checkout the master branch to avoid changes to this script
           # This is to reduce the possibilities of a secret leak from modifying