_reindex and frozen indices

[Mekk]

I tried to reindex frozen index. In case of globb-ed (*) value it was silently ignored, in case of explicit full name it was rejected as non-existing.

It would be nice if frozen indices could be reindexed, or at least if attempt to reindex them generated clear error message

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Reindex)

[markwalkom]

Also requested here https://discuss.elastic.co/t/how-to-reindex-from-an-frozen-index/208801/2

[henningandersen]

Our plan is to remove the (old) frozen indices, see #70192 for current status. Hence we are closing this, since when that effort completes, the suggested change would no longer be useful.