Removed Unused Availability Attributes in Functions

[yakovmanshin]

• Since Package.swift already limits Functions support to Catalyst 13, macOS 10.15, tvOS 13, and watchOS 7, explicit checks for these versions aren’t needed
• @available attributes and #available checks for iOS 13 remain in place

[ncooke3]

I think these may still be relevant for other distributions like the Firebase zip distribution and maybe CocoaPods.

[paulb777]

I believe we're ok without these for CocoaPods, as well, except for maybe the Catalyst ones. However, I'm leaning towards keeping the full list for clarity.

[yakovmanshin]

Alright, sounds good.

Just one clarification. In v11, deployment target became iOS 13 for all components except Analytics and Crashlytics, but because SPM doesn’t support per-product versions, Functions (and other targets) continue using iOS 12 in Package.swift. Does that mean all features should fully support iOS 12, despite it’s been dropped?

[paulb777]

No, we're setting the expectations that only Analytics and Crashlytics are fully supported for iOS 12. For example the new implementation of FirebaseAuth has almost zero support for iOS 12.

[yakovmanshin]

Thanks!

Closing this PR.