Stable release plan for 4.0

There is: 🌱 Plan for SAML Authentication 4.x Active

That is: a plan, but no timeline.

I need to update the wording of it (after which I can close this issue), because it assumes that D10 support would need a 4.x branch. Which is clearly not the case, because 8.x-3.x supports D10 now.

japerry saved us from doom by preventing a 4.x release that was cut just to support D10. See https://medium.com/jakob-on-drupal/dont-go-making-major-version-changes-... for reasons, if interested.

So.... until the time some actual breaking architectural changes happen or we need to drop The 4.x branch is a red herring; it has not diverged from 8.x-3.x yet, and I fast-forward push it semi regularly.

The other issue's text has been updated now.