www/pocketbase: Update 0.36.9 => 0.39.9

While here, utilize GO_TARGET in favor of post-install target.

Changelog:
https://github.com/pocketbase/pocketbase/blob/v0.39.9/CHANGELOG.md

PR: 297111
Reported by: Christos Longros <chris.longros@gmail.com> (maintainer)
Approved by: osa, vvd (Mentors, implicit)
MFH: 2026Q3