Drop iOS 9-14 support

[herzbube]

libsgfc++ 2.0.1 contains changes (specifically: the use of std::filesystem::temp_directory_path()) which require the iOS SDK 13. This would require changing the deployment target from 9.0 to 13.0, i.e. dropping support for iOS 9-12.

However, according to the Wikipedia device support articles for iPhone, iPad and iPod the two iOS versions 13 and 14 do not drop support for any additional devices. This means we can change the deployment target directly to 15.0.

By changing the deployment target to 15.0 we drop support for the following devices:

• iOS/iPadOS 9
  • iPhone models: iPhone 4S
  • iPad models: iPad Mini 1st generation, iPad 2 and iPad 3rd generation
  • iPod models: iPod Touch 5th generation
• iOS/iPadOS 10
  • iPhone models: iPhone 5 and iPhone 5C
  • iPad models: iPad 4th generation
  • iPod models: None
• iOS/iPadOS 11
  • None
• iOS/iPadOS 12
  • iPhone models: iPhone 6, iPhone 6 Plus and iPhone 5S
  • iPad models: iPad Air 1st generation, iPad Mini 2 and iPad Mini 3
  • iPod models: iPod Touch 6th generation
• iOS/iPadOS 13 + 14
  • None

Apple discontinued the newest of these devices in 2016/2017, so 6-7 years ago. Although the list is quite long, this also provides a chance for getting rid of old cruft in the code. The planned jump to a new major version is also a good point in the project development cycle to announce such a change.