Bump syn from 2.0.13 to 2.0.15 in /ci/no-std-check (#630)

Bumps [syn](https://github.com/dtolnay/syn) from 2.0.13 to 2.0.15.
- [Release notes](https://github.com/dtolnay/syn/releases)
- [Commits](dtolnay/syn@2.0.13...2.0.15)

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

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