[Monitoring] date_histogram uses [interval] instead of [fixed_interval]

[pickypg]

Kibana version: 7.2.0

Elasticsearch version: 7.2.0

Describe the bug:

The Stack Monitoring UI is triggering a bunch of deprecation warnings in the logs:

[interval] on [date_histogram] is deprecated, use [fixed_interval] or [calendar_interval] in the future.

Steps to reproduce:

1. View any page with charts in Stack Monitoring.
2. Observe deprecation warnings in the logs.

Expected behavior:

No deprecation warnings.

[elasticmachine]

Pinging @elastic/stack-monitoring

[chrisronline]

This work was never backported, but I started the process now. The ES PR went into 7.2 so there is no reason to not backport this work.

[chrisronline]

These code changes have been backported