Clarify how to site-upgrade IEF 2.x => 1.x|3.x

I noticed this PR: https://git.drupalcode.org/project/inline_entity_form/-/merge_requests/103
which implies that the 3.x branch once released wont support drupal 9. I think it is very important to let folks know that this is
an additional reason to downgrade to 1.x rather than up to 3.x. Obviously many many of us aren't at drupal 10 yet.

Thanks...

@jwag956: Thanks for this information. Please state it there.
What do you think about php8.1?

re PHP 8.1 - no comment - I moved to PHP8 many months ago with no issues. Moving to D10 will take another few months as there are still modules we use that aren't quite ready.
FYI - this module is awesome and a critical part of one of our workflows for adding research publications with articles and authors - so we have 2 levels of inline forms... Thanks for all the hard work...