chore(MobileClient): Removing deprecated methods

[ruisebas]

Issue #, if available:

• Crash on AWSMobileClient when initialising at runtime.  #5458

Description of changes:
Removing the following methods from AWSMobileClient:

• interceptApplication(_:didFinishLaunchingWithOptions:)
• interceptApplication(_:didFinishLaunchingWithOptions:resumeSessionWithCompletionHandler:)
• setSignInProviders(_:)
• getCredentialsProvider()

These were meant to be removed way back in 2.11.0, but since they still exists in the Objective-C interface they were still accessible.

Check points:

• Added new tests to cover change, if needed
• All unit tests pass
• All integration tests pass
• Updated CHANGELOG.md
• Documentation update for the change if required
• PR title conforms to conventional commit style

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ruisebas]

Note: The AWSLex integration is failing because in order to run in Xcode 15 (i.e. the default in macos-13) we need to add a new flag -ld64. But this flag is not available in macos-12.

Because of the pull_request_target trigger, the workflow definition used for the integration tests is the one from main, so until we merge this PR they will still run in macos-12

[ruisebas]

This is needed for Xcode 15, which is the default now in macos-13