Use jack2-1.9.21-r1.ebuild from main gentoo portage tree as base for jackdbus-2.21...

Use jack2-1.9.21-r1.ebuild from main gentoo portage tree as base for jackdbus-2.21.0.ebuild

jack2 and jackdbus codebases were separated recently.
As part of LADIOS project at least, media-sound/jack2 will continue
to provide libjack and libjackserver, but neither
jackdbus nor jackd will be installable through the ebuild.

jackdbus will be installable via new media-sound/jackdbus ebuild.

jackd was deprecated in LADI project more than a decade ago
and soon will not be installable anymore (unless someone
creates in a different overlay a jackd ebuild that uses the
media-sound/jack2[-classic,-dbus] and handles the support
in a different project)

dbus device reservation code in jack2 libjackserver will be
enabled no matter what [classic/dbus] use flags are supplied.

The dbus use flag used to enable jackdbus for jack2.
Since then, jackd got dbus reservation support.
A dedicated jackd ebuild that uses media-sound/jack2[-classic,-dbus]
could provide installable jackd with with dbus device reservation support.