Drop ruby requirement from gemspec

The ruby requirement doesn't make sense here as we have gems that can
support all kinds of versions. We've already had to undo this for Ruby
2.7 in ManageIQ#49, and I just hit it with Ruby 2.6 in manageiq-loggers. In the
end the goal here is to pin rubocop, and also provide styles that Target
a specific version, but not _prevent_ installing on other versions.

.github/workflows/ci.yaml

@@ -12,6 +12,7 @@ jobs:
     strategy:
       matrix:
         ruby-version:
+        - '2.6'
         - '2.7'
         - '3.0'
         - '3.1'

manageiq-style.gemspec

@@ -9,7 +9,6 @@ Gem::Specification.new do |spec|
   spec.description   = spec.summary
   spec.homepage      = "https://github.com/ManageIQ/manageiq-style"
   spec.license       = "MIT"
-  spec.required_ruby_version = Gem::Requirement.new(">= 2.7.0")
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["rubygems_mfa_required"] = "true"