Running GitHub Workflows duplicate events: push and pull_request

The GitHub Workflow template runs for each set of samples on `push` and `pull_request`.

Based on my reading of the [syntax documentation](https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions), pull_request is necessary for forks.

It looks like folks contributing from a branch in the repository would trigger duplicate tests. Looking at [some data](https://github.com/GoogleCloudPlatform/nodejs-docs-samples/actions/workflows/ai-platform-snippets.yaml?query=event%3Apush), I don't see signs of duplicate work but documented points like https://docs.github.com/en/actions/using-workflows/triggering-a-workflow#using-multiple-events indicate that there should be.

Action Items from this issue should be one of:
- Removing `push` as an event trigger from each config and the template
- Adding an in-line comment to each config and the template explaining why we use both

@sofisl could you provide some insight into why we have both triggering events in the config?