[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.
If this is a bug or security fix, could you label this PR @colleenmcginnis? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

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
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[colleenmcginnis]

Closing in favor of #45510.