mark __Bookmarks__ as deprecated

__Bookmarks__ was to be removed in 4.2 but the user was never warned
in 4.0, so marking the feature deprecated for 4.2 and will be fully
removed later

see issues#20297
Note the RT::Attribute id which contains the deprecated __Bookmarks__ syntax

Without it, any admin trying to track down the deprecated syntax will be
pretty frustrated.
Merge branch '4.2/deprecate-bookmarks-syntax'