Migrate from MediatR to Immediate.Handlers?

[Atulin]

https://github.com/viceroypenguin/Immediate.Handlers

Nice boost in performance, since they use source generators. We'll have to see if everything can be ported, though.

• [x] Handlers
• [x] Validation behaviour
• [x] Base handler though this one should probably be migrated to typed results anyway

[Atulin]

https://github.com/viceroypenguin/Immediate.Handlers/issues/37#issuecomment-1929445498

[Atulin]

Switched to martinothamar/Mediator instead, since the upgrade path was simpler https://github.com/Genfic/Ogma/commit/9c836b1a1d5625c1bb7e0301d37ca8005718312c