Resolve "Inform users of up to $500 offer for GCP account"

What does this MR do?

Adds a dismissable banner notifying users that they can get a $500 credit for Google Cloud Platform through GitLab.

The banner is dismissed permanently.

Are there points in the code the reviewer needs to double check?

• Overall code structure

Why is this MR needed?

To promote the GCP/GKE integration and bonus signup deal.

Screenshots (if relevant)

Cluster Index View

Cluster Create -> Login View

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #43469