Allow closing a write index of a data stream #70903
Description
Today when attempting to close a data stream a validation error is returned indicating that it is forbidden to close a write index of a data stream. The idea behind that is to ensure that a data stream always can accept writes. For the same reason deleting a write index is not allowed (the write index can only be deleted when deleting the entire data stream).
However closing an index isn't as destructive as deleting an index (an open index request makes the write index available again) and there are other cases where a data stream can't accept writes. For example when primary shards of the write index are not available. So the original reasoning for not allowing to close a write index isn't that strong.
On top of this is that this also avoids certain administrative operations from being performed. For example restoring a snapshot containing data streams that already exist in the cluster.
Relates to #70861