upgrade to buf/protovalidate

[jon-whit]

protoc-gen-validate is superseded in the community by the official buf/protovalidate package. The protovalidate package brings some nice additions that we should make use of. Namely, we can control the error message explicitly, and so this improves our API DX with error messages returned from the protobuf validation middleware layer.

See:

• https://buf.build/blog/protoc-gen-validate-v1-and-v2/
• https://github.com/bufbuild/protovalidate/tree/main