Help: Clarify when execute_process ENCODING UTF-8 became the default

In commit e782811cfe (execute_process: Change default ENCODING to UTF-8,
2024-09-13, v3.31.0-rc1~94^2) we implied this by the version ranges in
the older defaults.  State it explicitly.

Help/command/execute_process.rst

@@ -160,7 +160,7 @@ Options:
 
    Use the UTF-8 codepage.
 
-   This is the default.  See policy :policy:`CMP0176`.
+   This is the default since CMake 3.31.  See policy :policy:`CMP0176`.
 
  ``UTF8``
    Use the UTF-8 codepage.  Use of this name is discouraged in favor