Use report warning opening file written by newer Blender binary

handle_subversion_warning() was reporting with RPT_ERROR type, replaced
with RPT_WARNING.

RPT_ERROR would stop python scripts opening files written by newer
Blender binary with bpy.ops.wm.open_mainfile(), preventing further code
from running. This does not seem right since Blender itself still loads
the files.

Ran into this checking T100446 in 2.93.

Differential Revision: https://developer.blender.org/D15712

source/blender/blenkernel/intern/blendfile.c

@@ -453,7 +453,7 @@ static void handle_subversion_warning(Main *main, BlendFileReadReport *reports)
       (main->minversionfile == BLENDER_FILE_VERSION &&
        main->minsubversionfile > BLENDER_FILE_SUBVERSION)) {
     BKE_reportf(reports->reports,
-                RPT_ERROR,
+                RPT_WARNING,
                 "File written by newer Blender binary (%d.%d), expect loss of data!",
                 main->minversionfile,
                 main->minsubversionfile);