[Update] Manual prebuilt rule updates notice for older stack versions

[approksiu]

Description:
Add the following note to the upgrade notes eg for releases < latest-3 versions (please rephase appropriately):

• If you are upgrading to the version older than "latest-3" (eg latest version is 8.9, and you are upgrading to 8.5) - you will have to update prebuilt rules manually. If you want to get automatic updates - please consider upgrading to the latest version.

[jmikell821]

@approksiu just to clarify, what are all 8.x versions this notice should apply to?

[approksiu]

@jmikell821 we should add this note to 7.17 - 8.5 docs. Thank you!

[joepeeples]

Removing the Docset: Serverless label, because we don't need to add anything to serverless docs. Serverless is always on the latest version.