Scala 3 Support for sttp-mock-server

[oldstevekenobi]

Decided to give sttp-mock-server a go the other day but realised there's unfortunately no Scala 3 support for it yet which meant we had to use wiremock (no major issues with this but sttp-mock-server looks like it has less boilerplate for us).

Apologies if this is a duplicate or there's something blocking it that I'm unaware of but I couldn't find an existing issue or branch