Update Elastic-Agent Kafka settings

[belimawr]

This commit updates the documentation from the Kafka output on standalone Elastic-Agent settings by removing the options that are not supported any more.

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @belimawr? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[kilfoyle]

LGTM! Thanks for fixing these up @belimawr!

One question: We also have this Processors section in the docs for Fleet-managed agent. Should that section also be removed, or are the topics settings only being deprecated for standalone agent? I can look after removing the Fleet-managed section if we need that.

[kilfoyle]

I just noticed this PR, so I guess the settings are being removed for Fleet as well. I will update the Fleet-managed docs.