Update event list for workflow triggering. (#2665)

- When configured "pull_request[type: opened] + push", workflow not getting triggered when there is a change in PR code/commits when the branch is from the parent's forked repo.
- Configured only pull_request event as any change can only go through PR. 
- pull_request event by default happens when opened, synchronize (means whenever there is change/commit, sync happens), and reopened. So this should be good enough for our use cases.

More details in [approach doc](https://docs.google.com/document/d/1YNYUAlEUXTi4UZG5gEFZGQ7oe7LxNKh7FYSg2qtIUvI/edit#heading=h.1c7qt5y1jb9)

.github/workflows/unit-test.yml

@@ -1,10 +1,7 @@
 # This workflow builds and unit tests both main Java API and also build/data tools for metadata generation.
 
 name: Testing Java API
-on:
-  push:
-  pull_request:
-    types: [opened]
+on: pull_request
 jobs:
   java-test:
     runs-on: ubuntu-latest