Bump smallvec from 1.8.1 to 1.9.0 in /src/rust (#7383)

Bumps [smallvec](https://github.com/servo/rust-smallvec) from 1.8.1 to 1.9.0.
- [Release notes](https://github.com/servo/rust-smallvec/releases)
- [Commits](servo/rust-smallvec@v1.8.1...v1.9.0)

---
updated-dependencies:
- dependency-name: smallvec
  dependency-type: indirect
  update-type: version-update:semver-minor
...

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

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

Author: dependabot[bot]