Reduce thanos allocated cache

Reduce the thanos query allocated memory percent to avoid OOMing for
large queries.
2 jobs for <span class="ref-name">bjk/thanos_query_cache_math</span> in 3 minutes and 39 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #189681446
rspec

00:01:58

 
  Lint
passed #189681447
cookstyle

00:01:40