[Feature]: Migrate to GetEntityId() for Untiy 6+

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Feature Description

We need to feature gate + transition to GetEntityId() and ToULong to get our bits for ids.

Use Case / Motivation

Unity 6 has deprecated GetInstanceId() and transitioned to GetEntityId() in order to try to unify DOTS across the stack. We need to intelligently support this so consumers don't break when onboarding to new Unity versions.

Proposed Solution

Migrate our InstanceId concept to the new EntityId flow, as well as tests and anywhere else that might require upgrades.

Alternatives Considered

No response