Update favicon for Auth0

[lfdebrux]

What problem does this pull request solve?

I noticed that when signing in to forms-admin Auth0 was using the old black crown icon, rather than the blue from the GOV.UK rebrand.

This PR updates the asset Auth0 uses for the favicon when signing users in to our service.

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• Ensure that you consider the wider context.
• Does it work when run on your machine?
• Is it clear what the code is doing?
• Do the commit messages explain why the changes were made?
• Are there all the unit tests needed?
• Do the end to end tests need updating before these changes will pass?
• Has all relevant documentation been updated?

[github-actions]

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