[ML] Switch from "interval" to "fixed_interval" for aggs in datafeed configs

[droberts195]

Following on from elastic/elasticsearch#41906 the ML UI needs to switch from using "interval" to using "fixed_interval" in aggregations starting with version 7.2, otherwise it will generate deprecation warnings in Elasticsearch.

The docs for "fixed_interval" are in https://www.elastic.co/guide/en/elasticsearch/reference/master/search-aggregations-bucket-datehistogram-aggregation.html#_fixed_intervals

A potential problem here is that "fixed_interval" does not support week as a unit, so if the UI was previously allowing "interval" to be specified in weeks these now need to be translated to days. ("fixed_interval" doesn't support month, quarter or year either, but these are variable length intervals, so would already have been causing problems in versions before 7.2 if they were used.)

[elasticmachine]

Pinging @elastic/ml-ui

[sophiec20]

After team discussion, for 7.2 in the DATA FRAME UI, we will limit this to creating calendar_interval only and give the user a fixed selection of 1m, 1h, 1d, 1w, 1M, 1q, 1y (or minute, hour, day...).

The back-end will only support calendar_interval and fixed_interval and will not support interval. This removed risk of future bwc.

Therefore, if the user wishes to create a fixed_interval job, then this will be possible in dev tools.

[walterra]

In 7.3 you can now also use the UI's advanced editor mode to switch to fixed_interval. 🎉