Clarify user state after Instance SCIM deprovision

What does this MR do and why?

• A user who is deprovisioned via Instance SCIM has a state of ldap_blocked, which was not previously reflected in the API docs.
• To make the code a bit clearer, I've added an alias method called system_block that we can call with SAML or SCIM wants to block a user.
• This state naming is a bit off for SAML and SCIM; I've created an issue to update it: #388487

Screenshots or screen recordings

This is what an admin sees after users are deprovisioned via Instance SCIM:

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.