Bump html-minifier from 3.5.20 to 3.5.21 (#3015)

Bumps [html-minifier](https://github.com/kangax/html-minifier) from 3.5.20 to 3.5.21.
- [Release notes](https://github.com/kangax/html-minifier/releases)
- [Commits](https://github.com/kangax/html-minifier/compare/v3.5.20...v3.5.21

)

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