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