Save the version in VERSION.txt instead of VERSION

Fixes https://github.com/libsdl-org/SDL/issues/6558

CMakeLists.txt

@@ -2960,8 +2960,8 @@ set(EXTRA_CFLAGS ${_EXTRA_CFLAGS})
 
 # Compat helpers for the configuration files
 
-if(EXISTS "${PROJECT_SOURCE_DIR}/VERSION")
-  file(READ "${PROJECT_SOURCE_DIR}/VERSION" SDL_SOURCE_VERSION)
+if(EXISTS "${PROJECT_SOURCE_DIR}/VERSION.txt")
+  file(READ "${PROJECT_SOURCE_DIR}/VERSION.txt" SDL_SOURCE_VERSION)
   string(STRIP "${SDL_SOURCE_VERSION}" SDL_SOURCE_VERSION)
 endif()