pip: update edk2-pytool-extensions requirement from ~=0.20.0 to ~=0.21.8

Updates the requirements on [edk2-pytool-extensions](https://github.com/tianocore/edk2-pytool-extensions) to permit the latest version.
- [Release notes](https://github.com/tianocore/edk2-pytool-extensions/releases)
- [Commits](https://github.com/tianocore/edk2-pytool-extensions/compare/v0.20.0...v0.21.8)

---
updated-dependencies:
- dependency-name: edk2-pytool-extensions
  dependency-type: direct:production
...

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