[docs] Update stack-version to 9.0.1

[colleenmcginnis]

This is a temporary solution until we have a solution for elastic/docs-builder#737. I used the substitution key stack-version so we can easily search and replace across repos until there's a permanent, centralized solution.

[mergify]

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

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label