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Make the downsample action skip the non-time-series indices

As discussed in elastic#93123 (comment):

    ... we think the downsample action should skip backing indices that are 
   not part of a time series data stream. We do a similar approach with the 
   unfollow action where non follower indices are skipped. And this allows to 
  have the same policy for both regular data streams and tsdb data streams.

Closes elastic#93123