Add known issue that Beats do not recover from transient network errors

[cmacknz]

See elastic/beats#40705

• Relates Add known issue that Beats do not recover from transient network errors  beats#40767

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @cmacknz? 🙏
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 looking after this @cmacknz!

[kilfoyle]

P.S. I added a backport-8.15 so this will go into the 8.15 docs.