Bump socket.io, gatsby and gatsby-plugin-postcss in /apps/docs

Bumps [socket.io](https://github.com/socketio/socket.io) to 4.7.1 and updates ancestor dependencies [socket.io](https://github.com/socketio/socket.io), [gatsby](https://github.com/gatsbyjs/gatsby) and [gatsby-plugin-postcss](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-postcss). These dependencies need to be updated together.


Updates `socket.io` from 3.1.1 to 4.7.1
- [Release notes](https://github.com/socketio/socket.io/releases)
- [Changelog](https://github.com/socketio/socket.io/blob/4.7.1/CHANGELOG.md)
- [Commits](socketio/socket.io@3.1.1...4.7.1)

Updates `gatsby` from 3.15.0 to 5.13.6
- [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/[email protected]@5.13.6)

Updates `gatsby-plugin-postcss` from 4.15.0 to 6.13.1
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-postcss/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/[email protected]/packages/gatsby-plugin-postcss)

---
updated-dependencies:
- dependency-name: socket.io
  dependency-type: indirect
- dependency-name: gatsby
  dependency-type: direct:production
- dependency-name: gatsby-plugin-postcss
  dependency-type: direct:production
...

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