[v10.2.x] Update JWT auth page

[grafana-delivery-bot]

Backport c912cb3 from #87523

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JWT is not applicable/available to Grafana Cloud. Only on-premise.

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[ephemeral-instances-bot]

Error building instance: Contact #proj-ephemeral-hg-instances if it is not a compile error. Logs

Error message

handling pull request closed event: deleting instance by slug: unexpected response status: status=502 responseBody=

<title>502 Server Error</title>

Error: Server Error

The server encountered a temporary error and could not complete your request.

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