Add GitHub Actions CI

[s3cur3]

Adds GitHub Actions for:

• build & test for all combinations of Elixir 1.10, 1.11, 1.12, 1.13, 1.14, and 1.15, plus OTP 22, 23, 24, 25, and 26
• basic code quality checks (and a formatting fix and unused dependency removal to match)
• checking for retired Hex dependencies

Also updates the maintainers list and URL to reflect the new Felt org.