[metricbeat] Remove deprecated fields

[roncohen]

We have numerous fields that have been deprecated. They should be removed.

• Docker Docker: remove deprecated fields #27933
• kafka
• kubernetes Remove deprecated fields from k8s module #28046
• redis Redis: remove deprecated fields #28246
• nats

This is not an exhaustive list. Part of this work is to find more deprecated fields and remove them. We also need to discuss if we can remove these fields in a minor.

[exekias]

We need to come up with the full list of affected metrics.

About deprecating these in 7.x:

Docs show deprecation warnings for the affected metrics (example). Still some users may be using the deprecated metrics. I would suggest to check if we are providing good alternatives to all of them (probably yes), then it's probably safe to remove them.

[botelastic]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[botelastic]

Hi!
We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
Thank you for your contribution!