Change the following in the `flutter create` templates.  I didn't make any auto-migrations for existing apps because none seem that critical:
1. Turn on `ASSETCATALOG_COMPILER_GENERATE_SWIFT_ASSET_SYMBOL_EXTENSIONS` in iOS and macOS.
1. Turn on `BuildIndependentTargetsInParallel` in macOS template.  https://github.com/flutter/flutter/pull/125827/files#r1181817619 
1. Turn on `DEAD_CODE_STRIPPING` in macOS template. 
1. Set `ENABLE_USER_SCRIPT_SANDBOXING=NO` in iOS and macOS template.  `flutter` scripts don't work with this on.  This might require a migration in the future to explicitly turn this one off. However at least for now if the setting isn't present it defaults to `NO`.

Add migration for `LastUpgradeVersion` so users won't see these validation issues in Xcode.

Run migrator on all the example apps.  A few aren't Flutter apps so I edited them in Xcode.

Fixes https://github.com/flutter/flutter/issues/140253
See also https://github.com/flutter/flutter/issues/125817 and https://github.com/flutter/flutter/pull/90304.