codec: restore ffmpeg-next 9.0.0 (FFmpeg 8.1.2 stays)

The 8.1.0 downgrade broke the build and the audio export: 8.1.0's
typed video encoder has no set_color_primaries /
set_color_transfer_characteristic, and its older audio path sent
near-NaN samples into the AAC encoder (transcode_mp4 and the oak-task
export test both failed). ffmpeg-next 9.0.0 supports ffmpeg_8_0/8_1
(the vcpkg pin of 8.1.2#3 is unaffected) and carries ffmpeg_9_0 cfg
branches for the day FFmpeg 9 lands.

crates/oak-codec/Cargo.toml

@@ -25,6 +25,6 @@ oak-core = { path = "../oak-core" }
 # release/<crate-version>, and every such pairing is broken upstream
 # (9.0.0 -> FFmpeg 9.0 removed AVCodec fields; 8.1.0 -> FFmpeg 8.1 added
 # enum variants; 8.0.0 -> FFmpeg 8.0 renamed FF_PROFILE_*).
-ffmpeg-next = { version = "8", features = ["static"] }
+ffmpeg-next = { version = "9", features = ["static"] }
 # `std::error::Error` impls for the crate-internal error enum.
 thiserror = "2"