Fix default target Ruby version from 2.5 to 2.6

This PR fixes default Ruby version from 2.5 to 2.6.

In rubocop#10626 (RuboCop 1.29.1) , the default target Ruby version is 2.5. This is an unexpected
regression behavior and the default target Ruby version is expected to be 2.6.

changelog/fix_default_target_ruby_version.md

@@ -0,0 +1 @@
+* [#10629](https://github.com/rubocop/rubocop/pull/10629): Fix default Ruby version from 2.5 to 2.6. ([@koic][])

lib/rubocop/target_ruby.rb

@@ -5,7 +5,7 @@ module RuboCop
   # @api private
   class TargetRuby
     KNOWN_RUBIES = [2.5, 2.6, 2.7, 3.0, 3.1, 3.2].freeze
-    DEFAULT_VERSION = KNOWN_RUBIES.first
+    DEFAULT_VERSION = 2.6
 
     OBSOLETE_RUBIES = {
       1.9 => '0.41',
@@ -172,7 +172,7 @@ def find_version
         right_hand_side = version_from_gemspec_file(file)
         return if right_hand_side.nil?
 
-        find_minimal_known_ruby(right_hand_side)
+        find_default_minimal_known_ruby(right_hand_side)
       end
 
       def gemspec_filename
@@ -206,11 +206,13 @@ def version_from_array(array)
         array.children.map(&:value)
       end
 
-      def find_minimal_known_ruby(right_hand_side)
+      def find_default_minimal_known_ruby(right_hand_side)
         version = version_from_right_hand_side(right_hand_side)
         requirement = Gem::Requirement.new(version)
 
-        KNOWN_RUBIES.detect { |v| requirement.satisfied_by?(Gem::Version.new("#{v}.99")) }
+        KNOWN_RUBIES.detect do |v|
+          v >= DEFAULT_VERSION && requirement.satisfied_by?(Gem::Version.new("#{v}.99"))
+        end
       end
     end
 

spec/rubocop/cli_spec.rb

@@ -150,7 +150,7 @@ def and_with_args
     expect($stderr.string).to eq ''
     expect($stdout.string)
       .to eq(["#{abs('example.rb')}:3:1: E: Lint/Syntax: unexpected " \
-              'token $end (Using Ruby 2.5 parser; configure using ' \
+              'token $end (Using Ruby 2.6 parser; configure using ' \
               '`TargetRubyVersion` parameter, under `AllCops`)',
               ''].join("\n"))
   end