articles/problem-reports: cease recommending the 'patch' keyword

'Writing FreeBSD Problem Reports' pleads for use of a keyword:

    patch

– https://docs.freebsd.org/en/articles/problem-reports/#pr-writing-tips

Keywords 'patch' and 'patch-ready' remain in Bugzilla –
https://bugs.freebsd.org/bugzilla/describekeywords.cgi
– but should no longer be used. Both keywords are deprecated:

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=227147

Update the article:

* end the plea
* explain why the available keywords should not be used.

PR:                                  267176
Bug report: https://bugs.freebsd.org/267176
Mentors:               gjb, carlavilla
Reviewed by:           gjb
Approved by:           gjb
Differential revision: https://reviews.freebsd.org/D37086