Replace README references about gitter with slack

[kaylareopelle]

Many READMEs in this repo include the following line:
You can get involved by joining us on our [gitter channel][ruby-gitter] or attending our weekly meeting.

As Slack is now the preferred place to chat, let's bulk update the READMEs to reference Slack instead.

[github-actions]

👋 This issue has been marked as stale because it has been open with no activity. You can: comment on the issue or remove the stale label to hold stale off for a while, add the keep label to hold stale off permanently, or do nothing. If you do nothing this issue will be closed eventually by the stale bot.

[kaylareopelle]

This is blocked until #529 is merged.

[github-actions]

👋 This issue has been marked as stale because it has been open with no activity. You can: comment on the issue or remove the stale label to hold stale off for a while, add the keep label to hold stale off permanently, or do nothing. If you do nothing this issue will be closed eventually by the stale bot.

[scbjans]

@kaylareopelle this is still relevant; could you please re-open the issue, and apply the keep label?

[kaylareopelle]

You got it! Thanks, @scbjans

[xuan-cao-swi]

Some of the README are using You can get involved by joining us in [GitHub Discussions][discussions-url] or attending our weekly meeting., and some of them use You can get involved by joining us on our [gitter channel][ruby-gitter] or attending our weekly meeting. (ruby-gitter will be replaced by slack).

Do we need to change GitHub Discussions to slack channel or have both?

[kaylareopelle]

Hi @xuan-cao-swi, good question. In my opinion, including both would be ideal.