Review use of deprecated APIs in the Metal backend

[andyleiserson]

#5641 updates the Metal backend to use autogenerated bindings. Unlike the current bindings, the generator applies deprecation status from Apple's headers in the generated bindings, making explicit our use of some deprecated APIs.

We should revisit this after #5641 lands to check whether non-deprecated alternatives exist.

[teoxoy]

Firefox 148 has a minimum macOS requirement of 10.15 and wgpu had some users still running 10.12. From what I've seen Apple is quite eager to mark parts of the API as deprecated. If we still want to support 10.11 (first version of Metal) we can't stop using the deprecated APIs; if we decide to bump up our macOS requirement, we could.

[teoxoy]

Using MTLGPUFamily instead of MTLFeatureSet would be nice from a maintenance standpoint (the feature sets are not even listed anymore in Metal's feature set table pdf; you have to use the wayback machine) but requires bumping up our macOS requirement to 10.15 for no real benefit from the user's perspective.