[Xcode, Objective-C, Swift] Remove legacy Xcode compatibility

[ealeksandrov]

This is a rebase of #2860. Check it for earlier discussion and approvals.

• ebee5bf is a cleanup of Xcode 3 compatibility in ObjectiveC and Swift templates (already merged in Update Xcode.gitignore #3935 for Xcode)
  • related code is unnecessary since ~2011
• a404348 is a cleanup of Xcode 8 compatibility in Xcode, ObjectiveC and Swift templates
  • related code is unnecessary since ~2017

Links to documentation supporting these rule changes:

Please note that as of April 2024 all iOS and iPadOS apps submitted to the App Store must be built with a minimum of Xcode 15 and the iOS 17 SDK.
(https://developer.apple.com/ios/submit/)

Also see https://stackoverflow.com/a/53039267

[Coeur]

Xcode 9 was released on 12 Sep 2017.
It's been an eternity that Xcode 8 isn't allowed anymore to publish any app on AppStoreConnect.
We could have deleted those lines since 2018-2019 without second thoughts.

[wirecat]

@ealeksandrov Thank you for dusting off that PR!