Following the argument that breaking the invariant that PromQL does
not look ahead of the evaluation time implies a breaking change, we
still need to keep the feature flag around, but at least we can
communicate that the feature is considered stable, and that the
feature flags will be ignored from v3 on.

Signed-off-by: beorn7 <beorn@grafana.com>

This follows the line of argument that the invariant of not looking
ahead of the query time was merely emerging behavior and not a
documented stable feature. Any query that looks ahead of the query
time was simply invalid before the introduction of the negative offset
and the @ modifier.

Signed-off-by: beorn7 <beorn@grafana.com>

Signed-off-by: beorn7 <beorn@grafana.com>