ci: add v1.x branch to main-checks workflow

Prepare for v2 development by ensuring CI runs on the v1.x
maintenance branch. This branch will be created to receive
security and critical bug fixes while main becomes the v2
development branch.

Author: maxisbey

.github/workflows/main-checks.yml

@@ -5,6 +5,7 @@ on:
     branches:
       - main
       - "v*.*.*"
+      - "v1.x"
     tags:
       - "v*.*.*"