No longer record source migration failures in subscription order notes

[james-allan]

Fixes #3565

Changes proposed in this Pull Request:

This PR updates the WC_Stripe_Subscriptions_Legacy_SEPA_Token_Update class to make sure it doesn't attempt to migrate source objects which are cards.

Stripe have confirmed they don't intend on migrating Source cards and so the note we were adding to subscriptions is confusing.

Testing instructions

1. Checkout of Stripe 8.7.0
   • git checkout 8.7.0
   • npm run build
   • ❗ This is important because in 8.8 we updated express payment methods to use pm_xxx
2. Purchase a subscription product using an express checkout method like Google Pay.
   • Make sure to disable ECE if you have that feature flag enabled.
3. You should end up with a subscription with a source object.
   • You can verify either via the database or via the edit subscription admin screen.
4. Renew the subscription by choosing the Process Renewal action on the Edit Subscription screen or via the Scheduled Actions.
   • On develop, in the subscription's notes you should see the following note.
   • On this branch there should be no note.

^{Note added to a subscription after failing to migrate a source to a payment method.}

---

• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)

[annemirasol]

LGTM 🚢

✅ Verified that notes are created in develop.
✅ Verified that notes are no longer created in this branch.
✅ Verified that migration messages are logged instead.

Patch run results inside purple square. Pre-patch run immediately preceding.

"Source is a card" message in logs.