Stop bumping RUBY_PATCHLEVEL in release versions (ruby#15502)

[[Misc #21770]](https://bugs.ruby-lang.org/issues/21770)

Author: k0kubun

test/ruby/test_rubyoptions.rb

@@ -1319,4 +1319,10 @@ def test_free_at_exit_env_var
   def test_toplevel_ruby
     assert_instance_of Module, ::Ruby
   end
+
+  def test_ruby_patchlevel
+    # We stopped bumping RUBY_PATCHLEVEL at Ruby 4.0.0.
+    # Released versions have RUBY_PATCHLEVEL 0, and un-released versions have -1.
+    assert_include [-1, 0], RUBY_PATCHLEVEL
+  end
 end

tool/merger.rb

@@ -65,7 +65,8 @@ def version_up(teeny: false)
     if teeny
       v[2].succ!
     end
-    if pl != '-1' # trunk does not have patchlevel
+    # We stopped bumping RUBY_PATCHLEVEL at Ruby 4.0.0.
+    if Integer(v[0]) <= 3
       pl.succ!
     end