Bump js-yaml and gulp-imagemin in /src/main/content/antora_ui

Bumps [js-yaml](https://github.com/nodeca/js-yaml) and [gulp-imagemin](https://github.com/sindresorhus/gulp-imagemin). These dependencies needed to be updated together.

Updates `js-yaml` from 3.13.1 to 4.1.1
- [Changelog](https://github.com/nodeca/js-yaml/blob/master/CHANGELOG.md)
- [Commits](nodeca/js-yaml@3.13.1...4.1.1)

Updates `gulp-imagemin` from 6.1.1 to 9.2.0
- [Release notes](https://github.com/sindresorhus/gulp-imagemin/releases)
- [Commits](sindresorhus/gulp-imagemin@v6.1.1...v9.2.0)

---
updated-dependencies:
- dependency-name: js-yaml
  dependency-version: 4.1.1
  dependency-type: direct:development
- dependency-name: gulp-imagemin
  dependency-version: 9.2.0
  dependency-type: direct:development
...

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

Author: dependabot[bot]