Update auth0-migration.md

[BrJan]

Deleted limited availability banner since Auth0 is GA

What are you changing in this pull request and why?

Checklist

• Review the Content style guide so my content adheres to these guidelines.
• For docs versioning, review how to version a whole page and version a block of content.
• Add a checklist item for anything that needs to happen before this PR is merged, such as "needs technical review" or "change base branch."

Adding new pages (delete if not applicable):

• Add page to website/sidebars.js
• Provide a unique filename for the new page

Removing or renaming existing pages (delete if not applicable):

• Remove page from website/sidebars.js
• Add an entry website/static/_redirects
• Run link testing locally with npm run build to update the links that point to the deleted page

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Name	Status	Preview	Comments	Updated (UTC)
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[matthewshaver]

Thank you @BrJan