[Building Sync-ups Tutorial] fixes in "Editing and deleting a sync-up" chapter

[FredericRuaudel]

Hi!

Following my last PR, I'm currently reviewing the "Building Sync-ups" tutorial and so far, there is a lot of changes so I've splitted up into one PR per chapter to ease the review of them, continuing with the "Editing and deleting a sync-up" chapter here…

Changes Made:

• Replaced the deprecated .cornerRadius() method with the newer .clipShape(.rect(cornerRadius:)) modifier to ensure compatibility with the latest SwiftUI standards.
• Updated the delete button to use a .destructive role instead of .foregroundColor(.red) for better semantics.
• Fix compilation errors by using withLock to ensure safe updates to the shared syncUp state during editing.
• Add a step in SyncUpDetail tutorial to utilize a type-safe .syncUps key when updating the parent list after deletion confirmation action.
• Make some slight changes in the text of the tutorial documentation due to the removal of outdated commentary regarding delegate actions.