Only admins can search blocked, banned and unconfirmed users

What does this MR do and why?

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/346086

This excludes blocked, banned, and unconfirmed users from searches by non-admin users.

SQL query for User.without_forbidden_states scope

[13] pry(main)> puts User.without_forbidden_states.to_sql
SELECT "users".* FROM "users" WHERE "users"."confirmed_at" IS NOT NULL AND "users"."state" NOT IN ('blocked', 'banned', 'ldap_blocked')

Migration output

== 20220404183350 AddForbiddenStateIndexToUsers: migrating ====================
-- execute("SET statement_timeout TO 0")
   -> 0.0004s
-- execute("        CREATE INDEX CONCURRENTLY users_forbidden_state_idx ON users\n        USING btree (state)\n        WHERE (state <> ANY (ARRAY['blocked', 'banned', 'ldap_blocked']));\n")
   -> 0.0039s
-- execute("RESET statement_timeout")
   -> 0.0004s
== 20220404183350 AddForbiddenStateIndexToUsers: migrated (0.0056s) ===========

Rollback output

❯ rails db:rollback
== 20220404183350 AddForbiddenStateIndexToUsers: reverting ====================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:users, :state, {:name=>"users_forbidden_state_idx", :algorithm=>:concurrently})
   -> 0.0112s
== 20220404183350 AddForbiddenStateIndexToUsers: reverted (0.0120s) ===========

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