Added 3.21.x branch to depencedy auto update

Signed-off-by: Ihor Aleksandrychiev <ihor.aleksandrychiev@northern.tech>

Author: aleksandrychev

.github/workflows/update-deps.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        branch: [master, 3.24.x]
+        branch: [master, 3.24.x, 3.21.x]
     steps:
       - name: Checks-out repository
         uses: actions/checkout@v4