Bump to rules_proto 6.0.2, separate protobuf 21.7 #1623

This is in preparation for adding a `MODULE.bazel` file, which will bring in dependencies that use rules_proto 6.x. This change avoids that warning, and changing it now ensures that `WORKSPACE` builds are still compatible. As explained in the 6.0.0 release notes, rules_proto 6.x no longer depends directly on com_google_protobuf, so we need to import it separately. We're keeping the Protobuf version as 21.7 for now, because Protobuf 22.x requires C++14 at a minimum, which Bazel 6.x doesn't support by default: - https://protobuf.dev/news/v22/#c11-support - https://github.com/protocolbuffers/protobuf/releases/tag/v22.0 The downside is that, unline rules_proto-5.3.0-21.7, we now end up building (and occasionally rebuilding) `protoc` as part of our build. However, this also enables clients to try "Protobuf Toolchainization" to download precompiled `protoc` binaries. - https://github.com/bazelbuild/rules_proto/releases/tag/6.0.0 - https://docs.google.com/document/d/1CE6wJHNfKbUPBr7-mmk_0Yo3a4TaqcTPE0OWNuQkhPs/edit For now, Protobuf Toolchainization requires Bazel 6.5.0 or Bazel >= 7 and the `--incompatible_enable_proto_toolchain_resolution` flag: - https://bazel.build/reference/command-line-reference#flag--incompatible_enable_proto_toolchain_resolution I've got a working draft implementation in a separate branch, though that experiment is certainly not urgent.

I'd previously checked for an existing `protobuf` rule, not `com_google_protobuf`. D'oh!

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