Add admin option to remove 2FA on /users API endpoint
### Release notes
Adds the ability for admin users to remove a users 2FA through the Users API
### Problem to solve
As a GitLab admin I want to be able to build my own tooling to manager the user lifecycle, including the ability to remove 2FA for a requesting user programmatically.
### Intended users
* [Cameron (Compliance Manager)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#cameron-compliance-manager)
* [Sidney (Systems Administrator)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sidney-systems-administrator)
### User experience goal
A user with an admin API token should be able to use the API to remove 2FA for a user who has lost/forgotten their backup codes and primary token generator.
### Proposal
Allow an admin to `PUT /users/:id` with the parameter `"two_factor_enabled": false` to remove 2FA from a user. This will not change the requirement to enable 2FA inherited from a group or global level.
For example:
`curl --request PUT --header "PRIVATE-TOKEN: <your_access_token>" --data "two_factor_enable=false" "https://gitlab.example.com/api/v4/user/1234"`
### Further details
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### Permissions and Security
Admin only
### Documentation
https://docs.gitlab.com/ee/api/users.html#user-modification
### Availability & Testing
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### What does success look like, and how can we measure that?
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### What is the type of buyer?
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### Is this a cross-stage feature?
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issue