Fix audio_soloud codelab

[filiph]

Fixes and reenables CI for audio_soloud. Updates the minimum iOS and macOS deployment target to what pkg:flutter_soloud needs.

Pre-launch Checklist

• I read the Effective Dart: Style recently, and have followed its advice.
• I signed the CLA.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.

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[domesticmouse]

Please fix up deprecated warnings:

info - lib/audio/audio_controller.dart:67:14 - 'addGlobalFilter' is deprecated and shouldn't be used. Please, to manage global filters use SoLoud.filters instead. Try replacing the use of the deprecated member with the replacement. - deprecated_member_use

[domesticmouse]

Please fix CI warnings

[filiph]

On it. I needed to fix another (runtime) issue in parallel, but all looks good now (after the codelab_build run finishes, fingers crossed).

Marco (@alnitak) and I also updated the codelab text: https://docs.google.com/document/d/1PZbGEXGbXUVsIpEMsSKMR_7b8ok5Gyl-fqFPmqOy_8w/edit

[filiph]

Ok, this is now ready for review.

The salient changes are https://github.com/flutter/codelabs/pull/2251/files/852f32f5c168029e650fe5fd2321785633386247..7c9df89e93905e168340f3d6728a00458395b286 (the last commit is just rerunning codelab_rebuild, which updates a bunch of random files).

[domesticmouse]

On it. I needed to fix another (runtime) issue in parallel, but all looks good now (after the codelab_build run finishes, fingers crossed).

Marco (@alnitak) and I also updated the codelab text: https://docs.google.com/document/d/1PZbGEXGbXUVsIpEMsSKMR_7b8ok5Gyl-fqFPmqOy_8w/edit

For googlers, please see cl/687449390