Bump sockjs from 0.3.19 to 0.3.21

Bumps [sockjs](https://github.com/sockjs/sockjs-node) from 0.3.19 to 0.3.21.
- [Release notes](https://github.com/sockjs/sockjs-node/releases)
- [Changelog](https://github.com/sockjs/sockjs-node/blob/v0.3.21/Changelog)
- [Commits](sockjs/sockjs-node@v0.3.19...v0.3.21)

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

Author: dependabot[bot]