Update spec to require unstable features to be disabled by default

[gregsdennis]

Related to #1339, we need to add language that requires implementations to disable unstable features as the default behavior.

[gregsdennis]

In conjuction with #1340, does this mean that a schema with an unstable keyword is expected to error by default?

[jdesrosiers]

In conjuction with #1340, does this mean that a schema with an unstable keyword is expected to error by default?

Yes. Consumers would need to opt in to using they keywords.

[gregsdennis]

Unstable features are being extracted into the feature life cycle. See #1444