Replace the remaining schedulers with AgentSet functionality

[EwoutH]

This PR completes the migration from schedulers to AgentSet functionality across the mesa-examples repository for all regular (non-gis/-rl) examples. Key changes include:

• Replaced RandomActivation, SimultaneousActivation, and RandomActivationByType schedulers with appropriate AgentSet methods
• Updated Model.step() implementations to use AgentSet activation
• Removed references to schedule.steps, schedule.agents, and schedule.agents_by_type
• Updated agent addition/removal logic to work with AgentSets
• Adjusted data collection and visualization code to use Model.steps and Model.agents

For more details on migrating from schedulers to AgentSets, see the migration guide.

[rht]

Everything else LGTM.