Cleaning up warnings in Demo app

[mischreiber]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

Cleaning up existing warnings in the Demo app, and adding warnings-as-errors to catch any future ones that come up.

Changes:

• Removed unneeded deprecated override of prepareForInterfaceBuilder(), since DemoControllerScrollView is not used in Interface Builder anymore.
• Copied and used implementation of onChange_iOS17() to silence cross-version warnings (it's deprecated in visionOS 1.0, but the new version is not available on iOS 15).
• Enabled Objective-C and Swift warnings as errors in Demo app target.

Binary change

n/a - no changes to library

Verification

• Verified that the removed prepareForInterfaceBuilder() was never invoked in iOS or visionOS before removing.
• Ensured that theme changing still worked as expected.

Pull request checklist

This PR has considered:

• Light and Dark appearances
• iOS supported versions (all major versions greater than or equal current target deployment version)
• VoiceOver and Keyboard Accessibility
• Internationalization and Right to Left layouts
• Different resolutions (1x, 2x, 3x)
• Size classes and window sizes (iPhone vs iPad, notched devices, multitasking, different window sizes, etc)
• iPad Pointer interaction
• SwiftUI consumption (validation or new demo scenarios needed)
• Objective-C exposure (provide it only if needed)

Microsoft Reviewers: Open in CodeFlow