Updated recommended settings:
iOS app:

macOS app:

1. Asset symbol extensions ASSETCATALOG_COMPILER_GENERATE_SWIFT_ASSET_SYMBOL_EXTENSIONS seems harmless. Not worth a migration, but we could turn it on in the flutter create template.
2. Flutter runs on iOS 11, so we shouldn't update the target version until the minimum bumps to iOS 12. Ditto macOS
3. Should not turn on quoted headers Fix all "Double-quoted include in framework header, expected angle-bracketed instead" warnings #60025
4. Definitely shouldn't turn on ENABLE_USER_SCRIPT_SANDBOXING, Flutter xcode_backend is copying files around in derived data:

error: Sandbox: rsync.samba(24872) deny(1) file-write-create /Users/magder/Library/Developer/Xcode/DerivedData/Runner-gnengnkipgqsumexkvwkdczgvrst/Build/Products/Debug-iphoneos/Flutter.framework (in target 'Runner' from project 'Runner')
error: Sandbox: dart(24844) deny(1) file-write-create /Users/magder/Library/Developer/Xcode/DerivedData/Runner-gnengnkipgqsumexkvwkdczgvrst/Build/Products/Debug-iphoneos/.last_build_id (in target 'Runner' from project 'Runner')
error: Flutter failed to write to a file at "/Users/magder/Library/Developer/Xcode/DerivedData/Runner-gnengnkipgqsumexkvwkdczgvrst/Build/Products/Debug-iphoneos/.last_build_id".

5. BuildIndependentTargetsInParallel should be on in macOS and was already turned on in iOS. https://github.com/flutter/flutter/pull/125827/files#r1181817619
6. Dead code stripping seems good?

See #125817 and #90304 for previous cases of updating build settings for new Xcode versions.