Bump sockjs from 0.3.18 to 0.3.21

Bumps [sockjs](https://github.com/sockjs/sockjs-node) from 0.3.18 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@v0.3.18...v0.3.21)

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Author: dependabot[bot]