[v11] Use one Buf workspace instead of three

[ravicious]

Backport #19774.

Here I had a tricky conflict in genprotos.sh. There's a bunch of commits on master which add buf generate calls but those commits are not backported to v11 (compare commits between Aug 29, 2022 and Jan 9, 2023).

v11 does just buf generate at the moment. Before the signle workspace was introduced, this command would generate protos only for modules defined in the topmost buf.work.yaml which used to be just api/proto and proto, without prehog and teleterm.

The strategy I picked to solve this conflict was "Make it so that running make grpc and make grpc-teleterm doesn't introduce any changes" which boiled down to replacing buf generate with two separate calls for api/proto and proto.

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Another strategy I could've picked was "Bring v11 up to par with master". This would mean backporting the individual changes from those missing commits. There's not that many of them but at the same time those commits introduce other features/changes which might not end up being backported to v11. I could go around and ask but OTOH if the given team eventually decides to backport their commits, doing so should be straightforward.

The missing commits are related to:

• Making the Gogo/protoc-gen-go split: I'm not sure if that's something we need to backport to v11 or not.
• Device Trust: lib/devicetrust is not on v11 so I assume it won't be backported.
• Adding lib/prehog to make grpc: I'm not sure if the reporting team will need to be able to backport any proto changes to v11. The reason we introduced a single workspace in the first place was importing lib/prehog protos from lib/teleterm and I know that this most likely will not be backported into v11 and will land in v12.
• Connection Proxy: Since this change wasn't backported, I assume that the related change in genprotos.sh is also not needed in v11.