Using constants when checking for payment method detach

[wjrosa]

Changes proposed in this Pull Request:

This very simple PR just replaces 2 occurrences of payment method names with their constants introduced in https://github.com/woocommerce/woocommerce-gateway-stripe/pull/3449.

Testing instructions

Code review is enough.

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• [ ] Covered with tests (or have a good reason not to test in description ☝️)
• [x] 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)

[wjrosa]

Hey @wjrosa this PR looks good but I noticed there's more instances of 'stripe' and 'stripe_sepa' being referenced in this file.

For example, the exact same condition ~30 lines above this: https://github.com/woocommerce/woocommerce-gateway-stripe/pull/3581/files#diff-f80280bf89ae5350f2729f8c88d0d39cd4df34c7f13fe87db1445bcb03d6ac34R438

Did we want to update other cases in this file or leave it for now?

Approving as it's not blocking, more just curious to learn what alerted you to these non-constants being used and not others in the file 😅

Good catch, Matt! I have replaced all occurrences in https://github.com/woocommerce/woocommerce-gateway-stripe/pull/3581/commits/a4b37e360003b61824eb041d540dbe8abafcdf75