Update CocoaPods, support new Xcode, drop iOS 11 support

[liamnichols]

• Closes Bump cocoapods from 1.11.3 to 1.14.3 #17
• Closes Bump cocoapods from 1.11.3 to 1.15.2 #23
• Closes Use Xcode 14.1, 14.3.1 and 15.2 #19

This pull request updates the repository to support modern versions of Xcode (14 and 15). The following changes have been made:

• Bump iOS and tvOS minimum deployment version to 12
• Remove tvOS and watchOS support from CocoaPods (use SPM instead)
• Update CI to use Xcode 14 and 15
• Update CocoaPods
• Refactor CI workflow to run tests in parallel