chore: Rename NEXTAUTH_URL as it is no longer required for Next-Auth

[bryan-robitaille]

Summary | Résumé

Renames the NEXTAUTH_URL parameter to HOST_URL as it is used in other parts of the application.

Future versions of Next-Auth (v5) have an issue when the NEXTAUTH_URL is present. Our current version (V4) does not explicitly require the NEXTAUTH_URL and can operate without it.

Infrastructure Dependency: cds-snc/forms-terraform#606

Test instructions | Instructions pour tester la modification

• Rename the NEXTAUTH_URL in you .env file to HOST_ENV
• Run the app with yarn dev or yarn build && yarn start

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• Have you completely tested the functionality of change introduced in this PR? Is the PR solving the problem it's meant to solve within the scope of the related issue?
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