Make sure we can build the library and run the tests using Xcode 14

[lawrence-forooghian]

Motivation

We need to be able to develop the SDK using Xcode 14, since macOS 13 does not run earlier Xcode versions. This means that new developers who are already on macOS 13, or existing developers who perhaps upgraded too hastily (yes, it's me), are unable to develop the library.

Terminology

Here, I'm using "Xcode 14" to mean "Xcode 14.something".

Requirements

1. Fix all issues that prevent us from building the SDK and running the tests using Xcode 14. So far, I've identified these ones, but there might be more:
   • Increase minimum supported OS versions to remove warnings when building with Xcode 14 #1511
   • Replace usage of SecKeychainItemDelete to fix deprecation error in Xcode 14 #1521
   • Fix error on trying to override HTTPURLResponse._allHeaderFields with Xcode 14 #1522
2. Switch CI to build using Xcode 14.

[sync-by-unito]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-2915

[maratal]

Tests are already using Xcode 14.2, so #1511 doesn't prevent them from doing it. Do we still need two issues (this and #1511)? @lawrence-forooghian

[lawrence-forooghian]

I think we can close this one now.