Rename default branch to 'main'

We've renamed the default branch to main, fix the GitHub Actions
workflows to match.

Author: lfdebrux

.github/workflows/publish.yaml

@@ -3,7 +3,7 @@ name: Publish
 on:
   push:
     branches:
-      - master
+      - main
   workflow_dispatch:
 
 concurrency: rubygems