multimedia/openh264: security update to v2.6.0

This includes a security fix:
"- Fix potential bug in the codebase (Commit: 63db555e, PR: #3818)"
which the 2.5.1 release described as
"Fix decoder heap overflow vulnerability".
https://github.com/cisco/openh264/releases

But due to the other fixes, let's move to 2.6.0 right away.
Requires gmp-api (GeckoMediaPlayer) API update to Firefox 135 to build.

Changelog: https://github.com/cisco/openh264/blob/openh264v2.6.0/RELEASES#L4
Security: 03ba1cdd-4faf-11f0-af06-00a098b42aeb
Security: CVE-2025-27091
PR: 288375
Approved by: ports-secteam@ (fernape@)
MFH: 2025Q3 (needs gmp-api update)