Migrate to Connect RPC #1337
Description
Migrate our internal communication from gRPC to Connect RPC. We can/should do this in several steps
### Tasks
- [ ] https://github.com/clouditor/clouditor/issues/1339
- [ ] Replace gRPC-REST-Gateway with https://github.com/connectrpc/vanguard-go
- [ ] Remove gRPC protocol and switch to pure connect RPC protocol
- [ ] Remove gRPC dependencies
- [ ] https://github.com/connectrpc/connect-go/issues/694