Clean-up private variables (`_agents`, `_step`)

[EwoutH]

To keep backwards compatibility with Mesa 2.x, we added some attributes as private ones, like _agents and _step. We should clean that up for 3.0.

See:

• fix: Initialize model _steps and _time during __new__ #2026

[EwoutH]

Partly done in #2251 (agents_ removal) and #2223 (_time removal and _steps renamed to steps).

[quaquel]

What's still remaining?

[EwoutH]

It’s mentioned in the migration guide, but I think we want to enforce this somehow.

[EwoutH]

That would actually be

• Handling Private/Reserved Variables in Mesa #2230

So closing this one.