Use push event instead

create-pull-request doesn't work with pull-request event.

https://github.com/peter-evans/create-pull-request/issues/75

.github/workflows/autobump.yml

@@ -2,7 +2,9 @@ name: autobump
 on:
   schedule:
     - cron:  '*/15 * * * *'
-  pull_request: # for testing
+  push: # for testing
+    branches:
+      - '**'
 
 jobs:
   autobump: