Replace gorilla/mux with Gin

[magnusbaeck]

Description

The github.com/gorilla/mux package seems to be a less than ideal choice these days. It lacked maintainers for a while but apparently has a team of maintainers since July 2023, but there still haven't been a release since 2020. I think we should replace it with the more actively developed Gin.

Motivation

There appears to be more active work going on in Gin. Having used both I haven't seen any particular drawbacks with Gin and it supports OpenTelemetry tracing out of the box. Even if gorilla/mux picks up the pace again I don't see any particular reasons for sticking to it.

Exemplification

N/A

Benefits

We'll use a more obviously maintained package.

Possible Drawbacks

Probably nothing except the migration cost.