Add "lintservemux" linter

### Your feature request related to a problem? Please describe.

The `lintservemux` linter ensures the the default `http.ServeMux` is not used and instead a custom router is created.

https://github.com/reillywatson/lintservemux

### Describe the solution you'd like.

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### Describe alternatives you've considered.

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### Additional context.

_No response_