Migrate user milestone logic

[pablonyx]

Description

• User Manager create function is not called by our user_db upon creating users via auth flow
• on_after_register is a more reliable way to track user sign up progress
• Relevant Docs: https://fastapi-users.github.io/fastapi-users/latest/configuration/user-manager/?h=on_after_reg#on_after_register

How Has This Been Tested?

Tested with personal Posthog account and :

• Google Auth
• Cloud
• Basic auth

Accepted Risk (provide if relevant)

N/A

Related Issue(s) (provide if relevant)

N/A

Mental Checklist:

• All of the automated tests pass
• All PR comments are addressed and marked resolved
• If there are migrations, they have been rebased to latest main
• If there are new dependencies, they are added to the requirements
• If there are new environment variables, they are added to all of the deployment methods
• If there are new APIs that don't require auth, they are added to PUBLIC_ENDPOINT_SPECS
• Docker images build and basic functionalities work
• Author has done a final read through of the PR right before merge

Backporting (check the box to trigger backport action)

Note: You have to check that the action passes, otherwise resolve the conflicts manually and tag the patches.

• This PR should be backported (make sure to check that the backport attempt succeeds)

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Name	Status	Preview	Comments	Updated (UTC)
internal-search	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 18, 2024 0:02am

[yuhongsun96]

:P why remove?