Bump proc-macro2 from 1.0.67 to 1.0.68 (#3363)

Bumps [proc-macro2](https://github.com/dtolnay/proc-macro2) from 1.0.67 to 1.0.68.
- [Release notes](https://github.com/dtolnay/proc-macro2/releases)
- [Commits](dtolnay/proc-macro2@1.0.67...1.0.68)

---
updated-dependencies:
- dependency-name: proc-macro2
  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>