change thresholds for unused dijet bb, cc, qq decfiles

Updated thresholds for 490000{6,7,8}{0,2,3,5}.py, to remove overlapping thresholds.

This means that we have changed the dec file name and updated gen level cuts.

These dec files have not been used for any simulation yet, so having asked on mattermost, I think updating them like this is fine.

However, let us know if you would rather we used new numbers.

Some of these files may fail in the Gauss stage, pending the new developments associated with unhadronised gluons from @philten

@mbradley could you check this for me when you get a moment - if you are happy, I'll remove WIP and request the merge.