Protobuf 3.23 is incompatible, so can't upgrade

[ejona86]

Protobuf 3.23 appears to be incompatible[1] with Protobuf 3.22. If we upgrade, users won't be able to upgrade gRPC.

gRPC 1.55 is the first version that used 3.22, so I think the best thing is for us to revert, in the released version, to the previous version we were using, which was 3.21.7.

protocolbuffers/protobuf#11986

[ejona86]

(On the plus side, we still haven't been able to get C++ compilation of our plugin working. #10048 We upgraded Java without upgrading C++. Reverting to the older version "fixes" the misalignment.)

[ejona86]

The point of the downgrade is to allow our downstream to upgrade to 3.23 as needed.

[ejona86]

I mixed up the incompatibility. The incompat is if you use a new codegen with old runtime. There's no way that could work; codegen can choose to use new features of the runtime. That's the same as in grpc.