[Security] Bump highlight.js in /Rnwood.Smtp4dev/ClientApp

Bumps [highlight.js](https://github.com/highlightjs/highlight.js) from 9.18.1 to 9.18.5. **This update includes a security fix.**
- [Release notes](https://github.com/highlightjs/highlight.js/releases)
- [Changelog](https://github.com/highlightjs/highlight.js/blob/9.18.5/CHANGES.md)
- [Commits](highlightjs/highlight.js@9.18.1...9.18.5)

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