Consider deprecating/ removing support for wildcards in `term` queries on `_index`.

[jtibshirani]

When evaluating a terms query against the _index field, simple wildcard patterns are accepted: https://gist.github.com/jtibshirani/7992dc355b677e8537c333616d3c0270

This is not a huge problem because index names can't contain asterisks, but I think it's confusing and not consistent with the normal behavior of terms queries. It would be good to consider deprecating + removing this ability. We support wildcard queries on the _index field, so the behavior could be changed without a regression in functionality.

[elasticmachine]

Pinging @elastic/es-search-aggs

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)