ci: restrict release workflows to main branch only

Add if: github.ref == 'refs/heads/main' to both release / create-pr
and release / pre jobs. Running from a feature branch will skip
immediately.

Author: tylerslaton

.github/workflows/stable-release.yml

@@ -30,6 +30,7 @@ env:
 
 jobs:
   create-release-pr:
+    if: github.ref == 'refs/heads/main'
     runs-on: ubuntu-latest
     timeout-minutes: 15
     steps: