CI: Migrate the welcome bot to GitHub Actions

[seisman]

Since the start of the project (efabb47), we have used the welcome bot to greet first-time contributors when they open an issue or submit a pull request. It has worked well, but the welcome bot has been unmaintained for five years.

This PR replaces it with the actions/first-interaction workflow, which is actively maintained and likely more lightweight — the welcome bot is a Probot app, whereas actions/first-interaction runs as a GitHub Action.

One limitation of actions/first-interaction is that it cannot comment when a pull request is merged. For now, I’ve combined the newPRWelcomeComment and firstPRMergeComment messages into pr_message. We may implement the firstPRMergeComment functionality using gh pr comment triggered by a PR merge event. Edit: I don't think it's possible, since we can't know if a PR is someone's first PR.

I've tested the welcome workflow in my own fork. It works well, with a welcome comment at seisman#2 (the format of the message is fixed in 1bbfde2) and no comment at seisman#3.

TODO

• Initial implemention
• Decide if we want to implement the firstPRMergeComment functionality in this PR
• Improve the welcome messages
• Make the same changes to the GMT repository
• Uninstall the welcome bot App from the GMT org