Prune _source to only contain stored fields

Gerrit queries fetch only stored fields from _source. So, pruning
contents of _source to only contain those fields will help reduce
index sizes. Although doing this introduces some limitations as
described in [1], they don't apply for Gerrit uses-cases as we can
always recreate the indices from notedb.

On a Gerrit instance with ~4M changes, this change brings down the
size of changes index from ~40G to ~25G.

[1] https://www.elastic.co/guide/en/elasticsearch/reference/7.17/mapping-source-field.html#include-exclude

Change-Id: Iffab01a1c0121f9f99ae0f3d10bc40842deb0598