Add covered_by_store_credit translation in en.yml

[rainerdema]

Summary

This PR adds a new translation key 'covered_by_store_credit' in en.yml.
The new key provides a specific message to inform users when their order is fully covered by store credits and no additional payment method is required.
This change is connected to an update made in Solidus Starter Frontend: solidusio/solidus_starter_frontend#348

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[kennyadsl]

@rainerdema did you consider adding this key to the starter fronted instead?

[elia]

@rainerdema did you consider adding this key to the starter fronted instead?

This! I too think we should push translations out of core unless they're not strictly related to it.

[rainerdema]

@rainerdema did you consider adding this key to the starter fronted instead?

Sure @kennyadsl, but the starter frontend doesn't have a translations file and mainly uses those from the core at the moment.
Even though I'm not fond of this, since I only have to add one key, I've preferred to simply do it here with this small PR.
Let me know if there is another way to do this for the starter frontend.

[elia]

This translation can open the way for having them in the starter solidusio/solidus_starter_frontend#348 (review)

[rainerdema]

Closing, it will be addressed with #5212