Bump serde from 1.0.204 to 1.0.209

Bumps [serde](https://github.com/serde-rs/serde) from 1.0.204 to 1.0.209.
- [Release notes](https://github.com/serde-rs/serde/releases)
- [Commits](serde-rs/serde@v1.0.204...v1.0.209)

---
updated-dependencies:
- dependency-name: serde
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>