games/retroarch: Update 1.19.1 => 1.21.0

Changelogs:
https://github.com/libretro/RetroArch/blob/v1.21.0/CHANGES.md

Improve port:

• Fix RUN_DEPENDS.
• Fix X11_USE.
• Switch from qt:5 to qt:6 - default for upstream.
• Install etc/retroarch.cfg as sample.

PR: 287943
Approved by: Daniel Menelkir <dmenelkir@gmail.com> (maintainer, timeout 2 weeks, fix build)
MFH: 2025Q3
Co-authored-by: Vladimir Druzenko <vvd@FreeBSD.org>

(cherry picked from commit d81332eb57279b166aa50f781ed37a03565e2004)