Set minimal Ruby version to 2.7

That's the minimal version we already test against. Also RuboCop uses
2.7 as target version.

.rubocop_todo.yml

@@ -1,18 +1,11 @@
 # This configuration was generated by
 # `rubocop --auto-gen-config`
-# on 2023-08-26 05:38:06 UTC using RuboCop version 1.54.2.
+# on 2024-01-08 10:34:35 UTC using RuboCop version 1.59.0.
 # The point is for the user to remove these configuration records
 # one by one as the offenses are removed from the code base.
 # Note that changes in the inspected code, or installation of new
 # versions of RuboCop, may require this file to be generated again.
 
-# Offense count: 1
-# Configuration parameters: Severity, Include.
-# Include: **/*.gemspec
-Gemspec/RequiredRubyVersion:
-  Exclude:
-    - 'puppet-ghostbuster.gemspec'
-
 # Offense count: 1
 # This cop supports unsafe autocorrection (--autocorrect-all).
 # Configuration parameters: AllowSafeAssignment.

puppet-ghostbuster.gemspec

@@ -12,6 +12,8 @@ Gem::Specification.new do |s|
   s.description = 'Try and find dead code in Puppet receipts'
   s.licenses    = 'Apache-2.0'
 
+  s.required_ruby_version = Gem::Requirement.new('>= 2.7')
+
   s.files       = `git ls-files`.split("\n")
   s.executables = `git ls-files -- bin/*`.split("\n").map { |f| File.basename(f) }