lorder: add deprecation notice

[emaste]

Summary

lorder has not been used by the base system since 0e1e341b486cdf4769195ba1e5b3cb32e7387873 and hasn't really been necessary for a very long time.

Will make sure that there's essentially no use of lorder in ports via an exp-run before committing.

[emaste]

Seems I forgot to request the exp-run, now done in https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=259254