Bump serenity from 1c21a54 to 8d3d4a0 (#97)

Bumps [serenity](https://github.com/serenity-rs/serenity) from `1c21a54` to `8d3d4a0`.
- [Release notes](https://github.com/serenity-rs/serenity/releases)
- [Commits](serenity-rs/serenity@1c21a54...8d3d4a0)

---
updated-dependencies:
- dependency-name: serenity
  dependency-type: direct:production
...

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>