Support iOS 16

[andreyyudin]

Summary

• lower the iPhone and iPad deployment target to iOS 16
• provide compatibility implementations for APIs introduced after iOS 16
• preserve TLS 1.3, mutual authentication, SPKI pinning, ALPN, pairing, and stored-key security
• support secure TLS identities with the public Security APIs available in Xcode 16
• update documentation and compatibility checks

Validation

• protocol checks passed, including live pinned mutual TLS
• compatibility: 102 checks passed
• window restore: 41,499 checks passed
• remote rules: 123 checks passed
• settings window: 307,709 checks passed
• tour: 111,189 checks passed
• update policy: 124 checks passed
• Mac release build succeeded
• arm64 iOS build succeeded with deployment target iOS 16.0
• tested successfully on a physical iPhone running iOS 16

Closes #112

[Saffsanity]

From a pass over all the open pull requests (nothing is decided yet): several open PRs add iOS 17-only APIs that an iOS 16 target would reject (two-parameter onChange in #94, #150, #156, #158 and #161, AccessibilityNotification in #95, UITextInlinePredictionType in #147), so whichever lands second would need them wrapped; textually it meets #158 in six files, #161 in StreamScreen.swift, #94 in HostSettingsPanel.swift and #117 in ContentView.swift. #66 (and #72 and #73, built on it) guards the same Duo code in DuoPostureReader.swift with SILL_NO_DUO_SDK where this PR uses #if compiler(>=6.2), so only one of the two guards should remain.