Update discussion and social media links.

[kevinbowrin]

This PR removes the Twitter link, adds the Mastodon link, removes references to the mailing lists, and reorders the link targets.

PR Checklist:

• [x] All new features have been tested
• [x] All new features have been documented
• [x] I have read the CONTRIBUTING.md file
• [x] I will abide by the code of conduct