Incus: Project restriction `restricted.containers.privilege=isolated` bypassable by omitting `security.idmap.isolated`

Incus is a system container and virtual machine manager. Prior to version 7.3.0, project-level enforcement of `restricted.containers.privilege=isolated` can be trivially bypassed, allowing a user to create a non-isolated (shared host idmap) container in a project that is configured to forbid them. The restriction only rejects an explicitly set `security.idmap.isolated=false` (or empty) and fails to enforce anything when the key is omitted entirely. Because an unset `security.idmap.isolated` defaults to `false` (non-isolation), a user simply leaves the key out and obtains exactly the container state the restriction is meant to forbid. This defeats the tenant-isolation guarantee the restriction exists to provide. Containers in the project share the host uid/gid map instead of receiving unique, non-overlapping ranges, weakening the isolation boundary between co-tenant containers and the host. Version 7.3.0 patches the issue.