Add known issue for 8.15.x memory increase.

[cmacknz]

• Relates Queue keeps stale event data in memory in 8.15 beats#41355

The same as elastic/beats#41379 but for agent instead.

[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.

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[kilfoyle]

LGTM! Just a small suggested fix.

Thanks a lot for adding this @cmacknz!

[lucabelluccini]

Thanks @cmacknz, @pierrehilbert, @kilfoyle for the prompt action.

[kilfoyle]

@cmacknz For this to show up in the 8.15 docs we'll need to backport into the 8.x, 8.16 and 8.15 branches. I can look after that unless you explicitly didn't intend to merge into those.

[cmacknz]

Just forgot to add the label, thanks!

[kilfoyle]

I added the other labels (8.x and 8.16) which will make sure it shows up in later 8.x releases as well.