Time limit the deprecation of :unique and :locked.

Time limit the deprecation of :unique and :locked.

The :unique and :locked attributes have had no effect since 5.8.8
and 5.005 respectively. They were deprecated in 5.12. They are now
scheduled to be deleted in 5.28.

There are two places the deprecation warning can be issued:
in lib/attributes.pm, and in toke.c. The warnings were phrased
differently, but since we're changing the warning anyway (as we
added the version of Perl in which the attributes will disappear),
we've used the same phrasing for this warning, regardless of where
it is generated:

    Attribute "locked" is deprecated, and will disappear in Perl 5.28
    Attribute "unique" is deprecated, and will disappear in Perl 5.28