macos: change our minimum version to macOS 13

macOS 12 is officially EOL by Apple and the project only supports
officially supported versions of macOS. Once publicly released, users on
older macOS versions will have to use older released builds.

macos/Sources/Ghostty/Ghostty.TerminalSplit.swift

@@ -447,19 +447,6 @@ extension Ghostty {
             }
 
             window.makeFirstResponder(to)
-
-            // On newer versions of macOS everything above works great so we're done.
-            if #available(macOS 13, *) { return }
-
-            // On macOS 12, splits do not properly gain focus. I don't know why, but
-            // it seems like the `focused` SwiftUI method doesn't work. We use
-            // NotificationCenter as a blunt force instrument to make it work.
-            if #available(macOS 12, *) {
-                NotificationCenter.default.post(
-                    name: Notification.didBecomeFocusedSurface,
-                    object: to
-                )
-            }
         }
     }
 }