Bug 27770: ES: Deprecated aggregation order key [_term] used, replaced by [_key]

Bug 27770: ES: Deprecated aggregation order key [_term] used, replaced by [_key]

Starting in Elasticsearch 6.0 _key should be used instead of _term to order
buckets by their term

To test:
1) Run prove t/db_dependent/Koha/SearchEngine/Elasticsearch/Search.t
2) If you observe an error about types, apply patch for bug 25669
3) Run prove t/db_dependent/Koha/SearchEngine/Elasticsearch/Search.t
4) Observe if you get a deprecation warning about order key
5) Apply patch
6) Run prove t/db_dependent/Koha/SearchEngine/Elasticsearch/Search.t
7) Observe no deprecation warning about order key
8) Sign off

Sponsored-by: Lund University Library
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>