build: bump gorilla/mux and drop gorilla/context

What does this MR do?

Bump gorilla/mux to latest so that gorilla/context can be dropped

Why was this MR needed?

• newer gorilla/mux is using native go context
• gorilla/context is deprecated/archived (see https://github.com/gorilla/context)

What's the best way to test this MR?

What are the relevant issue numbers?