Bump engine.io and gatsby in /client

Bumps [engine.io](https://github.com/socketio/engine.io) to 6.2.1 and updates ancestor dependency [gatsby](https://github.com/gatsbyjs/gatsby). These dependencies need to be updated together.


Updates `engine.io` from 4.1.2 to 6.2.1
- [Release notes](https://github.com/socketio/engine.io/releases)
- [Changelog](https://github.com/socketio/engine.io/blob/main/CHANGELOG.md)
- [Commits](socketio/engine.io@4.1.2...6.2.1)

Updates `gatsby` from 3.14.6 to 5.5.0
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/compare/gatsby@3.14.6...gatsby@5.5.0)

---
updated-dependencies:
- dependency-name: engine.io
  dependency-type: indirect
- dependency-name: gatsby
  dependency-type: direct:development
...

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