build(deps-dev): update rdoc requirement from 6.11.0 to 6.12.0

Updates the requirements on [rdoc](https://github.com/ruby/rdoc) to permit the latest version.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](ruby/rdoc@v6.11.0...v6.12.0)

---
updated-dependencies:
- dependency-name: rdoc
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile

@@ -12,7 +12,7 @@ group :test do
 end
 
 group :development do
-  gem "rdoc", "6.11.0"
+  gem "rdoc", "6.12.0"
 
   gem "rubocop", "1.59.0", require: false
   gem "rubocop-minitest", "0.34.5", require: false