Log Message:
devel/ruby-google-protobuf: update to 3.12.1

Update ruby-google-protobuf to 3.12.1.

3.12.1	(2020-05-20)

Ruby

* Re-add binary gems for Ruby 2.3 and 2.4. These are EOL upstream, however
  many people still use them and dropping support will require more
  coordination. (#7529, #7531).

3.12.0	(2020-05-12)

Ruby

* [experimental] Implemented proto3 presence for Ruby. (#7406)
* Stop building binary gems for ruby <2.5 (#7453)
* Fix for wrappers with a zero value (#7195)
* Fix for JSON serialization of 0/empty-valued wrapper types (#7198)
* Call "Class#new" over rb_class_new_instance in decoding (#7352)
* Build extensions for Ruby 2.7 (#7027)
* assigning 'nil' to submessage should clear the field. (#7397)