Add libavif support for iOS

[freakboy3742]

Changes proposed in this pull request:

• Adds build instructions to add libavif to the compiled dependencies.
• Bumps cibuildwheel to 3.1.2 to pick up bug fixes and Python 3.14 support for iOS

[freakboy3742]

This is currently getting stuck on pyroma checks - see #9116.

[freakboy3742]

I've spent the morning trying to resolve the build error on x86_64 iOS simulators, and I've come to the conclusion that it can't be done (or, at least, it can't be done without significant extra effort).

x86_64 builds of libavif require the use of nasm, and as far as I can make out nasm doesn't produce iOS-compatible binaries. The binaries it produces are rejected at link time as "not an object file".

Given that support for x86_64 iOS simulators is nearing end-of-life (this year's macOS and Xcode major releases will be the last versions that provided support for x86_64), it doesn't seem worth the effort to resolve this issue for x86_64 simulators.

[freakboy3742]

The Windows ARM64 failure looks like a network outage.

[radarhere]

The plan from #8858 of using aom only for encoding and dav1d for decoding for the smallest wheel size wasn't working here for x86-64 on iOS, so we have instead used aom for both encoding and decoding on that environment, without aom optimisations for the CPU.