Adds migration guide

Author: mnmaita

release-content/migration-guides/rodio_0_21.md

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+---
+title: "Rodio 0.21 Update"
+pull_requests: [20323]
+---
+
+### Audio Feature Flags
+
+Audio format related features were reworked and now `symphonia` will be the default backend for most of them.
+
+By default, Bevy will enable the `vorbis` feature, which supports OGG/VORBIS files through `lewton`.
+
+If you are not using Bevy's default features, here's a list you can use for reference:
+
+- `vorbis`: OGG/VORBIS audio format support (through `lewton`).
+- `wav`: WAV audio format support (through `symphonia`).
+- `mp3`: MP3 audio format support (through `symphonia`).
+- `mp4`: MP4 audio format support (through `symphonia`). It also enables AAC support.
+- `flac`: FLAC audio format support (through `symphonia`).
+- `aac`: AAC audio format support (through `symphonia`).
+
+There are also "fallback" backend flags you can use for certain formats:
+
+- `fallback-flac`: FLAC audio format support (through `claxon`).
+- `fallback-vorbis`: OGG/VORBIS audio format support (through `symphonia`).
+- `fallback-wav`: WAV audio format support (through `hound`).
+
+Notice that OGG/VORBIS support through `symphonia` is not the default due to issues with buffering, reverb, looping and spatial audio. Check the following issues/PRs for additional context:
+
+- https://github.com/RustAudio/rodio/issues/775
+- https://github.com/RustAudio/rodio/pull/786
+
+The `audio-all` feature was added for convenience. It will enable all the available audio formats. Here's the full list and the corresponding backend for each format:
+
+- AAC, FLAC, MP3, MP4, and WAV: `symphonia`.
+- OGG/VORBIS: `lewton`.
+
+### Audio Traits
+
+`type DecoderItem` was removed from the `Decodable` trait. Now `rodio::Sample` is an alias for `f32`.
+
+### Android Related Features
+
+The `android_shared_stdcxx` feature was removed, as `cpal`'s `oboe-shared-stdcxx` feature was also removed in favor of Android NDK audio APIs.
+
+Keep in mind that if you are using `bevy_audio` the minimum supported Android API version is now 26 (Android 8/Oreo).