Binary reduction: remove `AttributedText` and unneeded `.modifyIf` statements

[mischreiber]

Platforms Impacted

• [x] iOS
• [ ] macOS

Description of changes

WIP: Cleaning up some unneeded stuff in our SwiftUI components to reduce binary size a bit.

• iOS 15 added AttributedString support to Text, so our custom AttributedText is no longer needed.
• We do a little more modifyIf calling than is strictly necessary. Removing one in Avatar and another in ListItem.

Binary change

Total increase: 18,744 bytes Total decrease: -71,176 bytes

File	Before	After	Delta
Total	31,061,888 bytes	31,009,456 bytes	🎉 -52,432 bytes

Full breakdown

File	Before	After	Delta
Avatar.o	599,920 bytes	615,512 bytes	⚠️ 15,592 bytes
FluentNotification.o	698,656 bytes	700,312 bytes	⚠️ 1,656 bytes
IndeterminateProgressBar.o	233,584 bytes	235,080 bytes	⚠️ 1,496 bytes
AvatarModifiers.o	44,016 bytes	42,448 bytes	🎉 -1,568 bytes
SwiftUI+ViewModifiers.o	123,512 bytes	119,472 bytes	🎉 -4,040 bytes
ListItem.o	274,344 bytes	267,128 bytes	🎉 -7,216 bytes
__.SYMDEF	4,823,240 bytes	4,811,336 bytes	🎉 -11,904 bytes
UIKit+SwiftUI_interoperability.o	46,448 bytes	0 bytes	🎉 -46,448 bytes

Verification

Ongoing! Will post videos once validation is complete, and once I'm sure I won't be adding any more.

Visual Verification

Before	After
Screenshot or description before this change	Screenshot or description with this change

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