Reject v1 API updates for (non-routed) multi-collection aliases

The intent of SOLR-13407 was to block all updates on aliases that represent multiple collections. Currently, this works for deletes, commits, and optimize requests. Users get a message like: Update request to non-routed multi-collection alias not supported.

But currently we still allow document adds/updates to go through.

We should either close this last hole, so no update requests are allowed, or document it so the behavior clearer.