Update filter cache stats for latest stats API

[cbcase]

The cache size and cache evictions graphs aren't showing up on master because the stats API changed the "cache" field to "filter_cache". This PR updates the appropriate references, so the graphs are now working again (well, locally for me :).

[lukas-vlcek]

Hey, thanks for this PR! I will do a new minor Bigdesk release with this fix included this weekend.

[lukas-vlcek]

@cbcase (cc @brusic ) Do you think you can point me to the version of ES in which this change was made? Is it since 0.90.1 or something like that? We need to update support matrix appropriately.
Thanks a lot.

[brusic]

The change is required on 0.90.1. Upgraded from 0.20, so I did not test on 0.90.0 (or master). 0.90.2 was released today, so I will try it out shortly.

[cbcase]

The change was made in this commit: elastic/elasticsearch@8467021#src/main/java/org/elasticsearch/index/cache/filter/FilterCacheStats.java

Based on the tags, it looks like the change was introduced in 0.90.0.

[lukas-vlcek]

Thanks.