Prepare gRPC instrumentation donation for release

[kaylareopelle]

Overall, the goal is to update the donated gRPC gem to match the other contrib projects' structure and tooling. This will allow us to use our existing CI that runs tests and releases gems with this new instrumentation.

TODOs:

• Create a gRPC-specific example to replace the Gruf example
• Move from Standard.rb to Rubocop and Rubocop-Performance to match our other gems
• Migrate the tests from RSpec to Minitest
• Add the gRPC gem to the relevant CI workflows (ci-instrumentation.yml, dependabot.yml)
• Add the gRPC gem to the opentelemetry-instrumentation-all gem
• Update the README to match our style
• Update the Changelog for compatibility with toys
• Update code of conduct to match OTel docs
• Remove RBS
• Remove conflicting Code of Conduct file

Relates to the code merged in #1164