fix: refactor codebase to protobuf-go v2

[noahdietz]

A number of things are done to the project, all in the name of moving away from the deprecated protobuf-go v1 module.

• We've replaced every usage of the protobuf-go v1 with the v2 drop-in equivalent
  • This includes both code generators, gapic and cli, as well as supporting libraries
• Regenerated the grpc service config Go proto with the protobuf-go v2 code generator
• Update all BUILD.bazel files with appropriate Go dependency changes
• Remove protobuf-go v1 from go.mod/go.sum
• Updated repositories.bzl based on go.mod/go.sum changes
• Switch showcase.bash to use the individual protobuf-go v2 and grpc code gen plugins
• Switch/add installs for protobuf-go v2 and grpc code gen plugins in CI workflow

[codyoss]

Thank you for this!