feat(payments-next): Cancellation flow: Update Churn intervention logic

[elizabeth-ilina]

Because

• Need to handle when customer clicks "cancel subscription" for an active subscription and give them a potential churn intervention offer.

This pull request

Returns Cancel Interstitial offer content if:

• custom content exists
• currentInterval matches current subscription plan interval
• the current plan interval has an upgraded plan of
• the customer is eligible for the upgraded plan of

Otherwise, returns Cancel churn content if:

• churn content exists for “cancel”
• customer is eligible to redeem

If neither exists, the customer will continue to Cancel Subscription page.

• Includes logging metrics to StatsD/BQ

Issue that this pull request solves

Closes #PAY-3370

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• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

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