Bump serenity from 0.11.7 to 0.12.1

Bumps [serenity](https://github.com/serenity-rs/serenity) from 0.11.7 to 0.12.1.
- [Release notes](https://github.com/serenity-rs/serenity/releases)
- [Changelog](https://github.com/serenity-rs/serenity/blob/current/CHANGELOG.md)
- [Commits](serenity-rs/serenity@v0.11.7...v0.12.1)

---
updated-dependencies:
- dependency-name: serenity
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>