ci: Use trigger event with base branch context

[calvincestari]

I noticed that the 'Check PR Metadata' CI check was not always running or failing with the following error:

GraphQL: Resource not accessible by integration (addComment)

I did some reading and from this issue it seems as if the correct event to base the action on is pull_request_target rather than pull_request because of the need to elevate permissions for the action. The following in the GitHub documentation for pull_request_target is the relevant section:

This event runs in the context of the base of the pull request, rather than in the context of the merge commit, as the pull_request event does.

• I added the reopened event to the types that would trigger this check too.

[svc-apollo-docs]

✅ Docs preview has no changes

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Build ID: 22c6c416e5d0eeb0be9ba95b

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[calvincestari]

Merging without review since this only changes a CI check.