Deprecate explicit data sharing in favor of survey questions and versioning

> I would propose another simpler solution:
> * Solve https://github.com/google/ground-platform/issues/1701 and https://github.com/google/ground-platform/issues/2221
> * Remove data sharing agreements altogether and let the organizer implement them as a question in the survey.
> * Changes to the question will be logged/tracked because of #2221.
>
> @kenstershiro @lecrabe for approval.

_Originally posted by @gino-m in [#2178](https://github.com/google/ground-platform/issues/2178#issuecomment-3089763428)_

This change would only apply to new surveys; existing surveys would continue to work as-is.