[FIX] mail: restore pre v11 note color

The gray-lighter was modified at 88881e9f to make it lighter (#F9F9 instead of
Restore the previous grey and increase contrast.

To quote "The Non-Designer's Design Book by Robin Williams":

> Contrast is created when two elements are different. If the two elements are
> sort of different, but not really, then you don't have contrast, you have
> conflict. That's the key–the principle of contrast states that if two items
> are not exactly the same, then make them different. Really different.

opw-1835722

addons/mail/static/src/less/thread.less

 @mail-thread-avatar-size: 36px;
 @mail-thread-icon-opacity: 0.6;
 @mail-thread-side-date-opacity: 0.6;
-@mail-thread-note: @gray-lighter;
+@mail-thread-note: #eee;
 
 .o_mail_thread, .o_mail_activity {
     .o_thread_show_more {