dependabot: Bump the cargo group across 1 directory with 2 updates (#48)

Bumps the cargo group with 2 updates in the /backend directory: [h2](https://github.com/hyperium/h2) and [mio](https://github.com/tokio-rs/mio).


Updates `h2` from 0.3.19 to 0.3.26
- [Release notes](https://github.com/hyperium/h2/releases)
- [Changelog](https://github.com/hyperium/h2/blob/v0.3.26/CHANGELOG.md)
- [Commits](hyperium/h2@v0.3.19...v0.3.26)

Updates `mio` from 0.8.8 to 0.8.11
- [Release notes](https://github.com/tokio-rs/mio/releases)
- [Changelog](https://github.com/tokio-rs/mio/blob/master/CHANGELOG.md)
- [Commits](tokio-rs/mio@v0.8.8...v0.8.11)

---
updated-dependencies:
- dependency-name: h2
  dependency-type: indirect
  dependency-group: cargo
- dependency-name: mio
  dependency-type: indirect
  dependency-group: cargo
...

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