Upgrade protoreflect from 1.9.0 to 1.15.1

[pbabbicola]

An older version of protoreflect imported google.golang.org/grpc/test/grpc_testing directly, instead of generating it or directly using it from google.golang.org/grpc/interop/grpc_testing. google.golang.org/grpc has removed it in the latest version (v1.55.0).

Unfortunately this makes old versions of protoreflect and therefore ghz incompatible with google.golang.org/grpc v1.55.0. Upgrading protoreflect would solve the issue.

I ran the tests locally, but there's two that are failing (TestRunClientStreaming/with_stream_cancel and TestRunBidi/with_stream_cancel). They are also failing on master for me, so maybe it's not important, but I thought i should mention it.

(There's also a linting error when I run the linter, from preexisting code. I can fix it also if you would like me to.)