Bump browserslist from 4.16.0 to 4.21.3 in /examples/grpc-web (#70)

Bumps [browserslist](https://github.com/browserslist/browserslist) from 4.16.0 to 4.21.3.
- [Release notes](https://github.com/browserslist/browserslist/releases)
- [Changelog](https://github.com/browserslist/browserslist/blob/main/CHANGELOG.md)
- [Commits](browserslist/browserslist@4.16.0...4.21.3)

---
updated-dependencies:
- dependency-name: browserslist
  dependency-type: indirect
...

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