Lifting proto2 native map restriction for Ruby

[RomanHargrave]

What version of protobuf and what language are you using?
Version: 3.13.0 (gentoo)
Language: Ruby

What operating system (Linux, Windows, ...) and version?

Linux, Gentoo. Kernel version not relevant.

What runtime / compiler are you using (e.g., python version or gcc version)

Ruby 2.7.0p0 MRI (647ee6f091)

What did you do?
Steps to reproduce the behavior:

1. Generate Ruby code for a proto2 file containing maps (the Murmur gRPC definitions in this case)
2. Observe the runtime error 'Cannot add a native map field using proto2 syntax.'

What did you expect to see

That native maps were supported.

What did you see instead?

The aforementioned error

Specific details

What work needs to be done on this? If this is straightforward and does not require extensive familiarity with protoc and the Ruby protobuf library, I would be glad to fix this.

Relevant code: https://github.com/protocolbuffers/protobuf/blob/master/ruby/ext/google/protobuf_c/defs.c#L1725

Ping @haberman (your name is in the TODO comment for this)

[haberman]

This is fixed in #8184