[CT-1916] Rename 'constraints_enabled' to 'contract'

[MichelleArk]

In the foundational dbt Constraints work, we added a boolean config named constraints_enabled that “turns on” contracting for this model. Given that the scope of model contracts is broader than “constraints,” and that dbt’s enforcement may go beyond the built-in behavior of analytical data platforms, we should rename this config to contracted instead.

[jtcohen6]

@sungchun12 in #6271 (comment):

I recommend the syntax be: contract: true | false because contracted invokes imagery of pregnancy(a beautiful human experience but dissonant related to data contracts) at first glance and is in past tense, which may confuse the reader. Whenever the term data contracts come up, it's a noun in plural or singular, not a verb.

@gshank in live conversation:

When "the economy has contracted," that's not a good thing! I like contracts better.

[jtcohen6]

I'd vote for contract (singular) over contracts (false), given some of the recent discussion in #6726 about how this is a "producer-side" contract, and there's going to be exactly one (or zero) per model.