nixos/dbus: switch default implementation to dbus-broker (#512050)

- The default kernel package has been updated from 6.12 to 6.18. All supported kernels remain available.

- The default D-Bus implementation has been switched from `dbus` to `dbus-broker`. dbus-broker provides

  higher performance and reliability while maintaining compatibility with the D-Bus reference implementation.

  Note that changing `services.dbus.implementation` is a **switch inhibitor**: switching between

  implementations requires a reboot rather than just `nixos-rebuild switch`, because restarting D-Bus

  mid-session is unsafe.

  Users who wish to keep the classic daemon can set: `services.dbus.implementation = "dbus";`

## New Modules {#sec-release-26.05-new-modules}

<!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->

          "dbus"

          "broker"

        ];

        default = "dbus";

        default = "broker";

        description = ''

          The implementation to use for the message bus defined by the D-Bus specification.

          Can be either the classic dbus daemon or dbus-broker, which aims to provide high