Migrate github.com/bufbuild/connect-go to connectrpc.com/connect

[DavidSGK]

As described on https://github.com/bufbuild/connect-go, this package has stopped being maintained and been moved. I was running into some issues trying to use some recently changed generated code so decided we should follow the recommendation and migrate.

Making changes here first since the CLI gets imported as a library in core.

On a related note, you can't really use both github.com/bufbuild/connect-go and connectrpc.com/connect at the same time, you get errors like this:

panic: proto: file "connectext/grpc/status/v1/status.proto" is already registered
       previously from: "connectrpc.com/connect/internal/gen/connectext/grpc/status/v1"
       currently from:  "github.com/bufbuild/connect-go/internal/gen/connectext/grpc/status/v1"
See https://protobuf.dev/reference/go/faq#namespace-conflict

which is why this is a wholesale change.