Add custom header text to billing page of free personal namespaces

What does this MR do?

Adds a text to the header of free personal namespace billing pages. The text directs users to buy a plan from a group's billing page.

Text: Looking to purchase or manage a subscription for your group? Navigate to your group and go to Settings > Billing.

This is one of the implementation MRs for #300345 (closed)

This MR is related to this discussion.

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Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• I have followed the style guides.