net/mtg: Update 2.2.6 => 2.2.8

Changelogs:

• https://github.com/9seconds/mtg/releases/tag/v2.2.7
• https://github.com/9seconds/mtg/releases/tag/v2.2.8

Port changes:

• Add pkg-post-install script to change default secret in the configuration file for security purposes.

PR: 294300
Reported by: Oleg Streejak <oleg@pcbtech.ru>
Approved by: Boris Korzun <drtr0jan@yandex.ru> (maintainer)
Approved by: osa (mentor)
MFH: 2026Q2

(cherry picked from commit 64b3e5969883b4679b3dc1dda34a1cdf6442ff85)