Bump github.com/gorilla/mux from 1.7.4 to 1.8.0

Bumps [github.com/gorilla/mux](https://github.com/gorilla/mux) from 1.7.4 to 1.8.0.
- [Release notes](https://github.com/gorilla/mux/releases)
- [Commits](https://github.com/gorilla/mux/compare/v1.7.4...v1.8.0

)

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