Deprecate App::add_system in favor of allowing single systems to be passed into add_systems #7992

What problem does this solve or what need does it fill?

There are several problems here:

1. It's easy to typo add_system as add_systems and vice versa.
2. The APIs are duplicative and somewhat confusing.
3. The APIs are annoying to refactor between.
4. Working with the complex mess of traits is a pain when writing app extension methods and internally.

What solution would you like?

1. Use the same trick as in [Merged by Bors] - Implement Bundle for Component. Use Bundle tuples for insertion #2975 and impl the relevant trait for single systems.
2. Deprecate App::add_system.
3. Remove all of the single system traits if possible.

This must have a deprecation notice to reduce the migration pain.

Additional context

Discussed on Discord with @cart.

We could do the same thing for plugins later. See #7631 for a parallel problem there.