Bump syn from 2.0.26 to 2.0.28

Bumps [syn](https://github.com/dtolnay/syn) from 2.0.26 to 2.0.28.
- [Release notes](https://github.com/dtolnay/syn/releases)
- [Commits](dtolnay/syn@2.0.26...2.0.28)

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

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