Bump Cocoapods minimum deployment target to iOS 13

[n8chur]

Bumps the Cocoapods iOS minimum deployment target of this framework to be in line with the iOS deployment target specified in the Swift Package.

• Bump minimum deployment target to iOS 13 in podspec
• Remove iOS 12-specific code from sourcecode
• Bump deployment target of example project
• Remove iOS 12-specific code from example
• Update README with iOS 13 bump information
• Use scene APIs to determine key window since UIApplication.shared.keyWindow was deprecated