Add support for --rbs_out

[stefanvanburen]

Originally in #4361, but we need to add our typical version checks and testing.

Ref: https://github.com/protocolbuffers/protobuf/releases/tag/v34.0
Ref: protocolbuffers/protobuf#15633

Supersedes #4361.

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf CI / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Mar 10, 2026, 6:51 PM

[stefanvanburen]

@doriable had mentioned having claude take a look at this; my prompt was:

look at gh pr view 4361; I think we want to support rbs_out, but first we need to upgrade the version of protoc we're testing (maybe this is in the Makefile somewhere)? Can we do that so we make sure we're testing it?

And it basically one-shotted this, although it also bumped the PROTOC_VERSION in the Makefile, which I thought about more and didn't think was necessary, so I followed up with:

do we actually need to bump the PROTOC_VERSION for this? do tests fail without it?

which caused it to revert the Makefile changes, and now we're here. (I don't see any obvious integration testing involving calling protoc with these args, so unless I'm missing something this should be how the others work and are tested?)

[doriable]

So the answer is yes and no, lol ^^"

Basically, PROTOC_VERSION ensures that this is the protoc version used when developing in bufbuild/buf through make and it also updates the WKTs. So in this case, we should update this version if we plan on writing tests for it, but it's not strictly necessary to update this version to support the features from the latest protoc (since it would invoke the Ruby codegen through the user's local protoc as a codegen plugin, effectively).

[doriable]

We can bump the PROTOC_VERSION for wkts, etc. as a follow-up, this looks good to me!