Revisit usage of version.stack

inspired by https://github.com/elastic/docs-content/pull/2222

we are using `{{version.stack}}` in the same way we used it in the old versioned docs in some cases.

this doesn't make sense in the new system, because it implies that something is only true in the latest version, when it might have been true since the _base_ version

example: https://github.com/elastic/docs-content/pull/2222/files#diff-07f5858632ade331374d8096f51946affacd54bd741f04658a5288db4c879a13

most areas for concern I believe are in the dev docs zone but we probably all need to audit this ... might apply to other version variables as well

## Actions
_added by @georgewallace_
- Need to look across documents to see if it is being used in the wrong context
- pull together list of potential docs