build(deps): bump serde_with from 3.20.0 to 3.21.0 in /dstack

Bumps [serde_with](https://github.com/jonasbb/serde_with) from 3.20.0 to 3.21.0.
- [Release notes](https://github.com/jonasbb/serde_with/releases)
- [Commits](jonasbb/serde_with@v3.20.0...v3.21.0)

---
updated-dependencies:
- dependency-name: serde_with
  dependency-version: 3.21.0
  dependency-type: indirect
...

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

Author: dependabot[bot]