Support Drupal >=10.3. Modernize the code

Thanks @claudiucristea for your work on this! Rather than commit to the 8.x-1.x branch, maybe this should take on a new branch as part of dropping Drupal 9 (and older 10) support? It would also be an opportunity to drop the "8.x-" prefix, and make this a safer, easier change for users upgrading, since any issues could be worked out in the new branch rather than impacting the (relatively stable) 8.x-1.x branch.

@quicksketch! It's good to have you around. Are you using OG 8.x with Backdrop?

> It would also be an opportunity to drop the "8.x-" prefix

👍

@claudiucristea I am +1 to make a 2.x release too, does that work for you too?

RE

Read-only class properties

I backed out of them in this issue because of a serialization problem 🐛 Error: Cannot initialize readonly property Drupal\og\Plugin\Field\FieldWidget\OgComplex::$membershipManager Active

OK, moved to new 2.x. I think this branch should be set as default branch and main development to be done against 2.x. Only some issues should be back ported to 8.x-1.x. I don't have enough permissions to change the default branch. @amitaibu?

@claudiu.cristea. Thanks. You already have `Administer releases`. However, when I try to set 2.x as recommended, it appears disabled.

I cherry-picked the version into the 2.x branch to move the needle a bit, I want to get some constraints using PHP attributes.