Make D9 support visible

A couple things going on here:

1. There had been a draft page specifically for D9, but it wasn't published in the menu. Subsequently, changes where made in the D8 page, so D8/D9 got out of sync. Additionally, some (but not all) of the D8 page expanded to reference D9. (The process+comments are like 95%+ the same...) Let's remove the old D9-specific draft and just use the same page.
2. If you're reading the D8 page, there's a missing step for D9 (#9 (closed)). Added this.
3. The "Requirements" and page-titles point to D8 specifically. Indicate that D9 is OK. (The label Drupal 8/9 does this without implying anything about D10.)

changed the description

ping @seamuslee @MikeyMJCO

approved this merge request

from me

mentioned in commit 9616f913

merged