security/openconnect: Update 9.12 => 9.21

Changelog:
https://www.infradead.org/openconnect/changelog.html

• Remove unnecessary GNU_CONFIGURE_MANPREFIX.
• Fix warnings from portclippy.
• Add P11KIT option.

PR: 296085
Approved by: zi (maintainer, timeout 2 weeks)
Sponsored by: UNIS Labs
Co-authored-by: Vladimir Druzenko <vvd@FreeBSD.org>
MFH: 2026Q2

(cherry picked from commit 9fdee93d72012d505371d25f8b5bc9a4242bde0c)