xdg-shell: relax maximized size constraints
In practice, no compositor is disconnecting clients with a protocol error when they don't obey the size dictated by the compositor in maximized state. It's very unlikely that any compositor would start doing so, as this would kill many existing clients. Moreover, some clients have a maximum size (e.g. dialogs), and the compositor is still allowed to mark them as maximized, in which case it's not clear how the client should react if the configured size exceeds the maximum size.
Update the protocol to reflect reality. Since we still want clients to obey the size in the general case, keep the "must".
An alternative would be to bump the version to specify that the error is sent from that specific version only. But maybe that's more of an orthogonal thing: either way the protocol needs to be updated to reflect reality for the current version.