Migrating from Angel to Angel3

WARNING, backup your existing code before proceeding with migration as the process cannot be reversed. Angel project can be upgraded to Angel3 by following the steps below:

1. Run dart pub outdated --mode=null-safety. Make sure all the packages except angel_* are upgradable.
2. Check that all angel_* packages can be upgraded by referring to Migrated Angel3 Packages.
3. Upgrade all angel_* packages manually in pubspec.yml by referring to NNDB Basic Starter Template or NNDB ORM Starter Template
4. Run dart pub upgrade --null-safety to upgrade the rest of the packages automatically.
5. Run dart migrate to do the migration.
6. Fix and resolve NNBD related warnings and errors in the code.
7. Replace all angel_* packages in pubspec.yml with angel3_* equivalent by referring to Migrated Angel3 Packages.
8. Fix and resolve import errors in your project by updating them to angel3_* packages.