chore(deps): bump serialize-javascript, @microsoft/sp-build-web and spfx-fast-serve-helpers

Bumps [serialize-javascript](https://github.com/yahoo/serialize-javascript) to 6.0.2 and updates ancestor dependencies [serialize-javascript](https://github.com/yahoo/serialize-javascript), [@microsoft/sp-build-web](http://aka.ms/spfx) and [spfx-fast-serve-helpers](https://github.com/s-KaiNet/spfx-fast-serve-helpers). These dependencies need to be updated together.


Updates `serialize-javascript` from 6.0.0 to 6.0.2
- [Release notes](https://github.com/yahoo/serialize-javascript/releases)
- [Commits](yahoo/serialize-javascript@v6.0.0...v6.0.2)

Updates `@microsoft/sp-build-web` from 1.20.2 to 1.21.1

Updates `spfx-fast-serve-helpers` from 1.20.2 to 1.21.1
- [Commits](https://github.com/s-KaiNet/spfx-fast-serve-helpers/commits)

---
updated-dependencies:
- dependency-name: serialize-javascript
  dependency-version: 6.0.2
  dependency-type: indirect
- dependency-name: "@microsoft/sp-build-web"
  dependency-version: 1.21.1
  dependency-type: direct:development
- dependency-name: spfx-fast-serve-helpers
  dependency-version: 1.21.1
  dependency-type: direct:development
...

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Author: dependabot[bot]