Michalw/adjust max shards per node

related to: https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/8968

We now have 8 hot nodes and indices are configured to have 6 shards with 1 replica (12 shards total). With total_shards_per_node: 3 we only need 4 nodes healthy to be able to allocate all shards from an index.