Help: Update 4.2 release note for Visual Studio 18.0 release

The `Visual Studio 18 2026` generator is no longer experimental.

Help/release/4.2.rst

@@ -42,9 +42,7 @@ File-Based API
 Generators
 ----------
 
-* The :generator:`Visual Studio 18 2026` generator was added.  This is
-  experimental and based on "Visual Studio 2026 Insiders" because this
-  version of VS has not been released.
+* The :generator:`Visual Studio 18 2026` generator was added.
 
 * The :generator:`FASTBuild` generator was added.