Remove unnecessary fallback code for requires_gem

[r7kamura]

This is not a significant change, just minor refactoring.

requires_gem was added in rubocop 1.63.0.

• Add new requires_gem API for declaring which gems a Cop needs rubocop#12186

rubocop-rails currently requires rubocop 1.75.0 or higher.

rubocop-rails/rubocop-rails.gemspec

Line 39 in cf897f3

s.add_dependency 'rubocop', '>= 1.75.0', '< 2.0'

Therefore, the code to maintain compatibility with rubocop versions below 1.63.0 is likely no longer necessary.

[r7kamura]

Or, perhaps it would be better to make more aggressive changes as follows:

diff --git a/lib/rubocop/cop/mixin/target_rails_version.rb b/lib/rubocop/cop/mixin/target_rails_version.rb
index b4240f5b2..606344144 100644
--- a/lib/rubocop/cop/mixin/target_rails_version.rb
+++ b/lib/rubocop/cop/mixin/target_rails_version.rb
@@ -4,9 +4,6 @@ module RuboCop
   module Cop
     # Common functionality for checking target rails version.
     module TargetRailsVersion
-      # Informs the base RuboCop gem that it the Rails version is checked via `requires_gem` API,
-      # without needing to call this `#support_target_rails_version` method.
-      USES_REQUIRES_GEM_API = true
       # Look for `railties` instead of `rails`, to support apps that only use a subset of `rails`
       # See https://github.com/rubocop/rubocop/pull/11289
       TARGET_GEM_NAME = 'railties' # :nodoc:
@@ -17,15 +14,6 @@ module RuboCop
         when String then requires_gem(TARGET_GEM_NAME, version)
         end
       end
-
-      def support_target_rails_version?(version)
-        return false unless gem_requirements
-
-        gem_requirement = gem_requirements[TARGET_GEM_NAME]
-        return true unless gem_requirement # If we have no requirement, then we support all versions
-
-        gem_requirement.satisfied_by?(Gem::Version.new(version))
-      end
     end
   end
 end

If so, please let me know and I will make the changes.