DEV: Clean up the workflow

After a bit of a trial and error I figured it out: it didn't trigger for tags that pointed to older commits because GitHub Actions runs workflows that are present in a given ref (and those didn't have any workflows yet)

.github/workflows/publish.yml

@@ -1,6 +1,9 @@
 name: Publish Gem
 
-on: [create, push]
+on:
+  push:
+    tags:
+      - v*
 
 jobs:
   build:
@@ -10,6 +13,5 @@ jobs:
 
       - name: Release Gem
         uses: cadwallion/publish-rubygems-action@5dfdb61
-        if: contains(github.ref, 'refs/tags/v')
         env:
           RUBYGEMS_API_KEY: ${{secrets.RUBYGEMS_API_KEY}}