FR: Limit DeviceCheck attestation provider availability in App Check to macOS versions where it is fully functional

### Feature proposal

* Firebase Component: App Check

Although the DeviceCheck APIs are available on macOS 10.15+, they are only usable as a functionality check (`DCDevice.current.isSupported` always returns `false`) until a later version of macOS. I think we should consider limiting the availability of the DeviceCheck attestation provider in App Check to macOS versions that truly support the feature.

I think that leaving it as 10.15+ could mislead developers into thinking App Check's DeviceCheck attestation provider is functional on Catalina, which may impact their decision to adopt it. This is especially true on macOS where new versions are adopted at a slower rate than iOS, requiring developers to target older versions for longer.

I'm unsure which version added supported but it was still nonfunctional as of macOS 11.3 (I [posted](https://developer.apple.com/forums/thread/682488) about it on the Apple Forums around WWDC 2021). It's working on macOS 12.5 but I haven't tested any of the later versions of Big Sur or earlier versions of Monterey so we'd need to narrow it down.

Note: This _may_ not be quite as easy as it seems to implement since DeviceCheck is the default provider. On macOS 10.15, custom providers are the only functioning solution AFAIK.