Drop code related to iOS 11 or below

[louwers]

Based on the discussion #689 we will only support iOS 12 or above going forward.

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• Remove simulators old iOS version in Makefile

https://github.com/maplibre/maplibre-gl-native/blob/95f41105e83746b28cc75886151be2aafbf2da0d/platform/ios/Makefile#L96

• Set iOS deployment target to 12.0 (the current deployment target of iOS 9.0 is not even supported by the newest version of XCode).

https://github.com/maplibre/maplibre-gl-native/blob/main/platform/ios/ios.cmake#L4

• Add note to README that only iOS 12 or above is supported

... If you know other places that need to be updated, let me know.

[birkskyum]

nitpick - For the readme, it's ">=12" that is supported, not " > 12".

[roblabs]

Based on this comment, we should make a mention of supported Xcode versions.

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@louwers — "(the current deployment target of iOS 9.0 is not even supported by the newest version of XCode)."

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Big Sur only supports up to Xcode 13.2.1. So maybe we can expect developers to use at least Xcode 13. The only mention (that I can recall) of Xcode requirements is in the badges/shields for maplibre/maplibre-gl-native-distribution.