Bump structopt from 0.3.21 to 0.3.22 in /cmd/pinniped-proxy (#3081)

Bumps [structopt](https://github.com/TeXitoi/structopt) from 0.3.21 to 0.3.22.
- [Release notes](https://github.com/TeXitoi/structopt/releases)
- [Changelog](https://github.com/TeXitoi/structopt/blob/master/CHANGELOG.md)
- [Commits](TeXitoi/structopt@v0.3.21...v0.3.22)

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

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