Drop openh264 extension point in 23.08
See the security issue mentioned in #1834 (closed) for more context. On 23.08 it is stuck to 2.2.0 due to actual ABI changes that happened and we cannot update to 2.6.0.
After discussion on Matrix, the consensus seems to be to drop it. I noted that we should wait for Mozilla to migrate to 24.08 which should be out on Flathub in v137. https://bugzilla.mozilla.org/show_bug.cgi?id=1925839
We need to announce as to why we are making an ABI break and dropping it, plus mention the migration path to ffmpeg-full.
I'll handle both.