Add stack and stack_next variables to assembler.yml

[colleenmcginnis]

I'm working on updating the docs release checklist for Elastic Stack releases. In the current draft, I'm recommending that the docs release coordinator touches this file twice:

• The day after feature freeze (after the new version branch is created): The docs release coordinator will update stack_next to the version of the upcoming release. The idea is that we'll be publishing those docs to staging (right?) so we'll be able to catch any build errors (like any broken cross-repo links) ahead of the release date. Does this make sense?
• The day of the release: The docs release coordinator will update stack to the new version and update stack_next to main (I don't think any Stack-versioned repos still use master).

Let me know what you think of this approach. I'm very open to suggestions.

[github-actions]

Label error. Requires exactly 1 of: automation, breaking, bug, changelog:skip, chore, ci, dependencies, documentation, enhancement, feature, fix, redesign. Found:

[Mpdreamz]

I am not loving staging being either main or the next M.N branch depending on the date.

This requirement might warrant on us explicitly having:

• current: aka production (9.0)
• next aka staging (9.1)
• preview. (main)

Content sources and environments.

That way the release manager can bump current/next the day off and we'd have confidence in next becoming current because it has been building all the time already.

@reakaleek @cotti wdyt?