Build(deps): Bump thewalrus from 0.20.0 to 0.21.0

Bumps [thewalrus](https://github.com/XanaduAI/thewalrus) from 0.20.0 to 0.21.0.
- [Release notes](https://github.com/XanaduAI/thewalrus/releases)
- [Commits](XanaduAI/thewalrus@v0.20.0...v0.21.0)

---
updated-dependencies:
- dependency-name: thewalrus
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>