[META] Prepare module for Drupal 9

I believe the only remaining task is to convert legacy tests to PHPUnit tests, but that doesn't prevent the module from working on Drupal 9. I think we're mostly ready!

I believe that if you add drupal 9 as an option to the composer.json file in this project you can get a nice little Compatible with Drupal 9 badge in the Project information section on the module page.

Below is a code sample from a module that has the badge.

"require": {
        "php": "^7.1",
       "drupal/core": "^8 || ^9"
  },

It also looks like it may possible to get the badge by adding the 'core_version_requirement' key in the module's info.yml file, which, in turn, will add the version to the require section of composer.json. Example follows.

core_version_requirement: ^8 || ^9

Does anyone know if this was already included in one of the sub-issues that were marked as fixed? If not, should we create a separate issue for it?

Adding the require version to composer.json file

Composer files are not required to specify the Drupal 9 compatibility information. It is enough to have a core_version_requirement in the info file (which is required anyway for Drupal to install it). Neither will show a "Drupal 9 badge" on the project page. You can edit your project and specify the "Drupal 9 porting info" value to make that show up with whatever text you find best describes your project readiness.

All related issues are fixed. #3134571: Drupal 9 compatibility is landed. Drupal 9 based test runs are green.

Thanks, everyone for making XMLSitemap module Drupal 9 compatible.

Any idea when we can get a new release so that folks can use an RC/stable vs. a dev release for D9? :)

+1. A release would be great :)

Please help review #3079398: Provide support for entity types without base tables, I'd like to get that ticket in before the next release.