Rename user state `ldap_blocked` to something more generic

Currently, users who are removed from LDAP are given a state of ldap_blocked, as the docs say:

The user is set to an ldap_blocked state in GitLab if the previous conditions fail. This means the user cannot sign in or push or pull code.

We also set a user's state to ldap_blocked if they are logging in with Group SAML but are not in the required SAML group.

Also also set a user's state to ldap_blocked if they are deprovisioned via Instance SCIM.

The naming of this state shows up in the UI for admins (these users with deprovisioned via Instance SCIM):

So, while this state is largely used for internal purposes, it is also shown to the end user. Given the naming, it is confusing that an instance without LDAP configured would have users with a state of ldap_blocked.

There is more context on how ldap_blocked is different from blocked in this MR where ldap_blocked was first added (before it was used for SAML and SCIM): gitlab-foss!2242 (comment 3181667)

Suggested change: change this state name to be something more generic, like system_blocked.