www/pmwiki: Update 2.5.8 => 2.6.0
While here, sort USES (pet portfmt).
Changelogs:
https://www.pmwiki.org/wiki/PmWiki/ChangeLog#v260
Release Notes:
https://www.pmwiki.org/wiki/PmWiki/ReleaseNotes#v260
PR: 296482
Approved by: osa, vvd (Mentors, implicit)
MFH: 2026Q2
(cherry picked from commit 0ad0daa2fb3f83403f8ac398d7a157f7ee0b3cb8)