Bump postcss and @angular-devkit/build-angular

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.19 and updates ancestor dependency [@angular-devkit/build-angular](https://github.com/angular/angular-cli). These dependencies need to be updated together.


Updates `postcss` from 7.0.17 to 8.4.19
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.17...8.4.19)

Updates `@angular-devkit/build-angular` from 0.802.0 to 15.0.0
- [Release notes](https://github.com/angular/angular-cli/releases)
- [Changelog](https://github.com/angular/angular-cli/blob/main/CHANGELOG.md)
- [Commits](https://github.com/angular/angular-cli/commits/15.0.0)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: "@angular-devkit/build-angular"
  dependency-type: direct:development
...

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