With reference to the filling of vacant posts for the position of Director-General and/or Head of Department in the Public Service, what (a) is the total number of (i) national and (ii) provincial departments which were required to advertise for the specified posts more than once during the (aa) 2018-19, (bb) 2019-20 and (cc) 2020-21 financial years, (b) are the reasons that the specified departments were required to advertise for the posts more than once and (c) total amount was spent on re-advertising the posts for each of the implicated departments?