Update CommandBar styling on iOS 18 to match iOS 26

[joannaquu]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

When the new CommandBar design was introduced, it was only brought to iOS 26. For coherence, bring it to iOS 18 as well.

Verification

Visual Verification

Before	After

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