[FIX] web_editor: restore crop widget backdrop color

Revert https://github.com/odoo/odoo/commit/14c8b285ef3e11cf8a8387bb211272e6456e4d42
which fixed a scss compilation error but changed the design by mistake.
The 4 digits hexa was not supported in old sass compiler so we use a
rgba() value instead.

closes odoo/odoo#52622

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/web_editor/static/src/scss/wysiwyg.scss

@@ -424,7 +424,7 @@ img.o_we_custom_image.mx-auto {
 }
 
 .o_we_crop_widget {
-    background-color: #888;
+    background-color: rgba(128, 128, 128, 0.5);
     @include o-position-absolute(0, 0, 0, 0);
     z-index: 1024;