Unwrap UPGRADING.md prose, link Ruby 4.0 release notes

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: dblock

UPGRADING.md

@@ -4,9 +4,7 @@
 
 ### Module renamed from `Ruby::Enum` to `RubyEnum::Enum`
 
-Ruby 4.0 reserves the top-level `Ruby` module for the language itself, so this
-gem's `Ruby::Enum` namespace is no longer safe to use. As of `2.0.0`, the gem
-uses `RubyEnum::Enum` instead.
+Ruby 4.0 [reserves](https://www.ruby-lang.org/en/news/2025/12/25/ruby-4-0-0-released/) the top-level `Ruby` module for the language itself, so this gem's `Ruby::Enum` namespace is no longer safe to use. As of `2.0.0`, the gem uses `RubyEnum::Enum` instead.
 
 `gem 'ruby-enum', '< 2.0.0'`
 
@@ -28,10 +26,7 @@ class Color
 end
 ```
 
-For backward compatibility, `Ruby::Enum` is still available as a deprecated
-alias for `RubyEnum::Enum` and will emit a deprecation warning when included.
-It will be removed in a future major version. Update your code to use
-`RubyEnum::Enum` and `RubyEnum::Enum::Case` directly.
+For backward compatibility, `Ruby::Enum` is still available as a deprecated alias for `RubyEnum::Enum` and will emit a deprecation warning when included. It will be removed in a future major version. Update your code to use `RubyEnum::Enum` and `RubyEnum::Enum::Case` directly.
 
 See [#55](https://github.com/dblock/ruby-enum/issues/55) for more information.