meson: install dbus configuration file in datadir
Install the org.freedesktop.bolt.conf not in sysconfdir but in datadir. The former is deprecated and the latter is the standard preferred way. I quote from the man page of dbus-daemon(1):
The standard system bus normally reads additional XML files from /usr/share/dbus-1/system.d. Third-party packages should install the default policies necessary for correct operation into that directory, which has been supported since dbus 1.10 (released in 2015).
Third-party packages would historically install XML files into /etc/dbus-1/system.d, but this practice is now considered to be deprecated: that directory should be treated as reserved for the system administrator.
Adapt the contrib/bolt.spec.in file accordingly.