docs: support only iOS 10 and above
docs: support only iOS 10 and above

- [#116](https://github.com/omise/omise-ios/pull/116) it already drop support for iOS 9
- [#113](https://github.com/omise/omise-ios/pull/113) it already drop support for iOS 8
- And 2 guidelines ([1](https://www.bot.or.th/Thai/AboutBOT/Activities/Documents/MediaBriefing_20122019.pdf), [2](https://www.bot.or.th/Thai/FIPCS/Documents/FOG/2562/ThaiPDF/25620311.pdf)) from Bank of Thailand (BOT) and should not allow to use obsolete operating system (OS) version.

omise/omise-ios,omise/omise-ios