Lower number of allowed POST requests to /users (sign up)

What does this MR do and why?

Related to https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/521.

Lower allowed POST requests to /users (sign up) per IP from 20/min to 20/day.

Why?

The current rate limit for POST requests to /users endpoint is currently too high (28,800 requests per day) and is actively being abused by attackers (see https://gitlab.com/gitlab-com/gl-infra/production/-/issues/17118 and https://gitlab.com/gitlab-com/gl-infra/production/-/issues/17141).

At the peak of the recent incidents (11-14 - 11-21) the highest number of requests to the endpoint from a single IP was < 500.

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