Update README for Ubuntu 24.04 unprivileged user namespace changes

Starting from v24.04, Ubuntu introduced new restrictions on the use of unprivileged user namespaces (see e.g. here). This is a breaking change for tor-browser-build, since the new restrictions prevent RBM from building and using containers. Disabling this new feature can be achieved by setting kernel.apparmor_restrict_unprivileged_userns=0. The README should be updated to reflect this.