ci: 👷 needs to push to main, not from PRs (#23)

## Description

The release workflow needs to push back to main, so can't use the
`on.pull_request` trigger.

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Author: lwjohnst86

.github/workflows/update-version.yml

@@ -1,9 +1,7 @@
 name: Update version
 
 on:
-  pull_request:
-    types:
-      - closed
+  push:
     branches:
       - main