Fix header on group billing page [RUN AS-IF-FOSS] [RUN ALL RSPEC]

What does this MR do?

This bug was reported in this comment.

The group billing header should consistently be relying on plan.name.

Screenshots (strongly suggested)

The group billing page makes an API call to CustomersDot to retrieve Plan data, through the FetchSubscriptionPlansService class. Plan names are now returned as they should be read on the billing page:

Before fix

After fix

Does this MR meet the acceptance criteria?

Conformity

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Security

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