Make search contexts GA (enable feature flag by default for enterprise deployments)

Created by: lguychard

We have been waiting for Enterprise customer feedback on search contexts before making them GA, but:

• We know that search contexts are the solution we want to pursue to replace version contexts
• They are also destined to replace repository groups, and should be made GA before repository groups are deprecated
• More generally, we feel like they are a convicing solution to enable users to specify subsets of their corpus to search over

As such, we may want to enable the feature flag by default for enterprise deployments immediately.