While site administrators cannot generally edit users in multisite, they have always been able to change the roles of users on their site. In the REST API however, this has not been possible so far. This changeset brings parity with how it is handled in the administration panel: A REST request to edit only a user's roles succeeds correctly, while a REST request to edit any further details of a user fails.
Props jnylen0.
Fixes#40263.
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