Added release note for RubyGems 4.0.3

Author: hsbt

NEWS.md

@@ -371,6 +371,7 @@ see the following links for details.
 * [4.0.0 Released - RubyGems Blog](https://blog.rubygems.org/2025/12/03/4.0.0-released.html)
 * [4.0.1 Released - RubyGems Blog](https://blog.rubygems.org/2025/12/09/4.0.1-released.html)
 * [4.0.2 Released - RubyGems Blog](https://blog.rubygems.org/2025/12/17/4.0.2-released.html)
+* [4.0.3 Released - RubyGems Blog](https://blog.rubygems.org/2025/12/23/4.0.3-released.html)
 
 ## Supported platforms