Only deploy the documentation from master

Don't do it for pull requests.

This check also seems to render the check for the repository owner
unnecessary, so drop it instead of keeping both.

.github/workflows/docs_update.yml

@@ -32,7 +32,7 @@ jobs:
           ./regen.sh php
 
       - name: Upload
-        if: github.repository_owner == 'wxWidgets'
+        if: github.ref == 'refs/heads/master'
         working-directory: docs/doxygen
         env:
           DOCS_WEBSITE_KEY: ${{secrets.DOCS_WEBSITE_KEY}}