Handle both V1 and V2 of Cirrus' data structure

[Vinnl]

getExperiments pretended that it always returned experiment data in the Cirrus V2 format, but in reality, it returned it in V1 format (by taking the Features property from the V2 format if that's what it got). The reason TypeScript didn't warn about this, is because response.json() resolves to an any, and thus we could just assert that to be the expected type even when it wasn't.

In this PR, I've updated it to either return the V2 format directly, or, if the CirrusV2 flag is disabled, return the V1 object wrapped in { Features: <the V1 object> }. And I created a new type ExperimentData_V2_Or_V2LikeV1 that reflects that, i.e. that is like the V2 format, expect that the Enrollments property can be undefined.

Once we remove the CirrusV2 flag, we can look at where that type is used and update it to just ExperimentData.

How to test

Well... I think we can only test this on stage, because I don't think we can run Cirrus locally. But we'll have to make sure to test both with the flag on and off :)

Checklist (Definition of Done)

• Localization strings (if needed) have been added.
• Commits in this PR are minimal and have descriptive commit messages.
• I've added or updated the relevant sections in readme and/or code comments
• I've added a unit test to test for potential regressions of this bug.
• If this PR implements a feature flag or experimentation, the Ship Behind Feature Flag status in Jira has been set
• Product Owner accepted the User Story (demo of functionality completed) or waived the privilege.
• All acceptance criteria are met.
• Jira ticket has been updated (if needed) to match changes made during the development process.
• Jira ticket has been updated (if needed) with suggestions for QA when this PR is deployed to stage.

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