chore(deps): update grpc requirement from >= 1.53, < 1.59 to >= 1.53, < 1.60 (#52)

Updates the requirements on [grpc](https://github.com/google/grpc) to permit the latest version.
- [Release notes](https://github.com/google/grpc/releases)
- [Changelog](https://github.com/grpc/grpc/blob/master/doc/grpc_release_schedule.md)
- [Commits](grpc/grpc@v1.53.0...v1.59.0)

---
updated-dependencies:
- dependency-name: grpc
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

flipt-grpc-ruby.gemspec

@@ -34,7 +34,7 @@ Gem::Specification.new do |spec|
 
   spec.require_paths = ["lib"]
 
-  spec.add_dependency "grpc", ">= 1.53", "< 1.59"
+  spec.add_dependency "grpc", ">= 1.53", "< 1.60"
 
   spec.add_development_dependency "bundler", "~> 2"
   spec.add_development_dependency "rake", "~> 13.0"