Drupal 11 compatibility fixes for views_accordion

Tests are all green:

patch to make it D11 compliant

I confirmed MR 7 can be installed in D11, and appears to work as expected.

📌 Automated Drupal 11 compatibility fixes for views_accordion Needs review ended up achieving the d11 goal. But this one has a lot of code standards and quality improvements. Let's rebase it and see if we can get it to a place for merge soon.

One note, why did we drop pre 10.1 support? I know we don't have any automated testing for previous versions, but are there specific reasons we're explicitly dropping prior releases?

I rebased the MR re-adding Drupal 9 support.