fix(payments-next): Reinforce subgroup order

[elizabeth-ilina]

Because

• While the subgroup order appeared correct in Strapi, query showed it out-or-order, which happens from time to time

This pull request

• Updates codebase to reinforce the order in the subgroup by adding a tier field to the offering type.
• If both from and target offerings have a tier, uses targetTier - fromTier
• If neither has one, falls back to array position, like before
• If only one has one, reports to Sentry, then fall back to position
• Strapi update PR:
• Precondition: this was checked in Strapi dev and stage, but need to check that there's no

Issue that this pull request solves

Closes #PAY-3844

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• 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).
• I have manually reviewed all AI generated code.

How to review (Optional)

To test the changes, in local Strapi, I made 3 offerings, VPN, 123 Foxkeh, 123donepro.
VPN and 123 Foxkeh are tier 10, 123donepro tier 20.
VPN and 123donepro are in a subgroup, 123Foxkeh is not in any subgroup.

Tested:

• Having vpn, purchasing 123:
  - Can successfully upgrade
  -

• Having 123donepro, purcahsing vpn:
  - "This subscription plan change is not supported" error page
  -

• Repeating above scnearios while switching the order of the subgroups in the offering to see if results were (correctly) consistent
  -

• Having vpn, getting foxkeh
  - foxkeh is regular checkout, since they're different subgroups

• Having foxkeh, getting 123donepro
  - regular checkout, since they're different subgroups

• have two offerings with the same tier in one subgroup (for ex., 123donepro and 123foxkeh make both tier 10)
  - expected: sentry error, customer just sees subscription switch invalid
  - is this expected? Or is it an existing/possible case to have two offerings with the same tier in a subgroup?

• Key files/areas to focus on:

• Suggested review order:

• Risky or complex parts:

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Other information (Optional)

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