libavif v1.2.1 release

[Yannis Guyon]

Hello everyone,

libavif v1.2.1 has been released. The source code of the release can be
checked out from the git repository[1] using the release tag v1.2.1.

[1] https://github.com/AOMediaCodec/libavif.git

Here is the change log:

1.2.1 - 2025-03-17

Added since 1.2.0
* Add support for outputting all frames of an image sequence in `avifdec`.
  `avifdec --index all sequence.avif out.png` creates files named
  `out-xxxxxxxxxx.png` where xxxxxxxxxx are the zero-padded frame indices.

Changed since 1.2.0
* Fix local libargparse dependency patch step on macOS 10.15 and earlier.
* Patch local libyuv dependency for compatibility with gcc 10.
* Use stricter C99 syntax to avoid related compilation issues.
* Update svt.cmd/svt.sh/LocalSvt.cmake to v3.0.1.