Drop iOS 12, 13 and 14 support (#2469)

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## Summary
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importantly what they aim to achieve -->
Drop support for iOS 12 and 13.

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## Link to pull request in Documentation repository
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Documentation: home-assistant/companion.home-assistant#

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Sources/Extensions/NotificationContent/NotificationLoadingViewController.swift

@@ -23,11 +23,7 @@ class NotificationLoadingViewController: UIViewController, NotificationCategory
 
         let activityIndicator: UIActivityIndicatorView
 
-        if #available(iOS 13, *) {
-            activityIndicator = UIActivityIndicatorView(style: .medium)
-        } else {
-            activityIndicator = UIActivityIndicatorView(style: .gray)
-        }
+        activityIndicator = UIActivityIndicatorView(style: .medium)
 
         view.addSubview(activityIndicator)
         activityIndicator.translatesAutoresizingMaskIntoConstraints = false