PR summary

Supersedes #30635
Related to #31043 (one will need a rebase after the other is merged)

Fixes the "PR Greeting" workflow by moving to the https://github.com/plbstl/first-contribution action as suggested by @bjlittle .

This action will:

• Post a comment to a first-contributor's PR, just like it did before.
• Add a "first-contributor" label to issues opened by users who had never opened issues in matplotlib/matplotlib before.

There is an option to add a message to first-time issues, but I'm not sure we need that. Also open to reformulate the first -time PR message if that's needed. One thing I had in mind is that maybe we want to point folks to the Discourse forum instead of Gitter?

Tests

I tested this on my fork. You can see the results here:

• PR MNT: add dummy issue message to PR greeting workflow melissawm/matplotlib#30
• Issue [MNT]: Test melissawm/matplotlib#32

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• [N/A] Documentation complies with general and docstring guidelines