Copy-pasted from flutter/flutter#130759:

Today (Jul 17, 2023), I renamed the good first contribution label to good first issue. This is because

1. good first issue is the more commonly used for this intent and may be (ever so slightly) more recognizable to potential new contributors, and, more importantly,
2. GitHub recognizes the label good first issue when surfacing good first issues in specific places in GitHub¹.

This PR updates Contributing.md to match the new name.

grep did not find any other references to good first contribution to update in this repo.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above. (not applicable in my opinion. If you'd like me to file an issue first, let me know what label would be appropriate.)
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt. not applicable: docs-only change
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.