Remove Needs: Submitter Input label on push

[jimchamp]

Closes #9199

Removes any existing Needs: Submitter Input Waiting on input from the creator of the issue/pr [managed] labels from PRs when the pull request's head branch is updated.

Technical

The pull_request_target event was chosen over pull_request event, as the workflow will not run for pull_request events if there is a merge conflict (more details here).

Our labeler configuration is set to add a Needs: Submitter Input Waiting on input from the creator of the issue/pr [managed] label if the PR includes changes to any file in the project's non-existent dev/null directory. When coupled with a truthy sync-labels input, the label is removed every time there is no change to any dev/null file. This isn't the most intuitive way to remove labels, but is recommended by the action's team (here).

.github/labeler.yml is the default path for the labeler's configuration file. If we want to move this elsewhere, the path is configurable (not that I'm suggesting that we move the file --- just pointing out that we can).

Testing

1. Merge this branch into the master branch of your repo (not this one)
2. Open a PR in your repo that makes a small change (maybe a README update)
3. Label the new PR Needs: Submitter Input. You'll likely have to create this label in your repo --- be sure to use the exact string when creating the label.
4. Update the PR. The Needs: Submitter Input label should be removed after the action finishes. Any other labels should be unaffected.

Screenshot

Stakeholders