feat(payments-next): Update email reminder logic

[david1alvarez]

Because:

• We need to send emails at 15 days prior to billing for annual plans
• We need to send emails at 7 days prior to billing for monthly plans that have an ending discount and are returning to full price

This commit:

• Updates the logic to check whether a customer has an persisting discount for a monthly plan, and does not send email reminders for those plans
• Renames the hadDiscount variable to discountEnding to better reflect its purpose

Closes #PAY-2291 (to follow after merge of PR in webservices-infra)

Checklist

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• I have verified that my changes render correctly in RTL (if appropriate).

[StaberindeZA]

r+ lgtm

Just the one nit that's optional.

Just a question regarding this note from the ticket...

NOTE: Take note of the current settings for Staging, as we typically send these emails sooner in Staging for testing purposes, and ensure they are set correctly for Daily test plans.

... is this PR meant to add functionality for the daily plan so it can be tested in Stage?

[StaberindeZA]

nit(non-blocking): update message type to match existing patterns

Typically the message type matches the method/function it's called from with some additional identifiers. This way it's easy to know just from looking at the message type where this log statement was called from.

Suggestion: subscription-reminders.skipping-monthly-no-discount => sendSubscriptionRenewalReminderEmail.skippingMonthlyNoDiscount