execute_process: Change default ENCODING to UTF-8

Windows is heading toward making UTF-8 the preferred MBCS.  As CMake's
internal encoding, `UTF-8` is effectively equivalent to `NONE`, which
was CMake's behavior prior to 3.15's accidental change to `AUTO`.
Behavior of `ENCODING UTF-8` is independent of CMake's internal
encoding, making it in principle a better default than `NONE`.

Add policy CMP0176 for compatibility and to document the default's
history.

Fixes: #26262

Tests/RunCMake/execute_process/Encoding.cmake

@@ -1,9 +1,12 @@
 if(CMAKE_HOST_WIN32 AND CODEPAGE)
+  cmake_policy(GET CMP0176 CMP0176)
+
   # Run cmake in a new Window to isolate its console code page.
   execute_process(COMMAND cmd /c start /min /wait ""
     ${CMAKE_COMMAND} -DTEST_ENCODING_EXE=${TEST_ENCODING_EXE}
                      -DENCODING=${ENCODING}
                      -DCODEPAGE=${CODEPAGE}
+                     -DCMP0176=${CMP0176}
                      -P ${CMAKE_CURRENT_LIST_DIR}/Encoding-windows.cmake)
 
   # Load our internal UTF-8 representation of the output.