[ML] Could not save changes when editing Anomaly Detection job's datafeed

[qn895]

Kibana version: main

Elasticsearch version: 8.1.0

Steps to reproduce:

1. Start a Anomaly detection job and start and stop the datafeed
2. Edit the job's datafeed then save

Expected behavior:
The new datafeed query should be saved

Screenshots (if relevant):

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context:

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[jgowdyelastic]

This has been broken by a change in the elasticsearch client in 8.0.0.
the updateDatafeed function in the client now ignores the body which is passed to it:
https://github.com/elastic/elasticsearch-js/blob/88933ad39e4ff7d8d3f74a1280e4d5fd00910055/src/api/api/ml.ts#L1936

Compared to 7.16 where the body is being used:
https://github.com/elastic/elasticsearch-js/blob/6020625912153a1b1c8d77c397a14f3dbb8f2b8b/api/api/ml.js#L1720

I wouldn't consider this a blocker for 8.0.0, as editing the datafeed for existing jobs isn't a high profile feature but it would be good to get it fixed for 8.0.0 if possible.
cc @sophiec20 @peteharverson

@delvedor would it be possible to get this fixed in the client for 8.0?

[jgowdyelastic]

API tests is missing for this endpoint.
I've added an item for to add tests for 8.0.0
#117930

[jgowdyelastic]

Raised js client issue
elastic/elasticsearch-js#1601