Remove gorilla/mux as a dependency

[JonCrowther]

The gorilla/mux library has been archived and hasn't been updated in a few years. In apiserver we use it for routing, but in Go 1.22 routing enhancements were introduced that cover our use case https://go.dev/blog/routing-enhancements.

This PR replaces all instances of gorilla/mux with the standard library net/http.

[pgonin]

will apiserver unit tests provide a sufficient non regression testing?

[JonCrowther]

will apiserver unit tests provide a sufficient non regression testing?

That's a good question. Maybe someone from @rancher/rancher-squad-frameworks would have a better feel for that