Replace explicit use of VectorMTJ by alias ActsTrk::TrackStateBackend
This is followup of !63356 (merged). It replaces use of explicit type from ACTS by an alias defined in Athena ActsTrk::TrackStateBackend.
After that one we should be only one step away from trying Athena backend just by recompiling the Reconstruction/Acts (+ modifications to how the MTJ gets constructed).
FI @pagessin @cvarni @ncalace @adye @mwolter (and please tag further if needed)
BTW. I think a similar action is needed for Acts::VectorTrackContainer