[meta] Drupal 10 compatibility tracking

As Drupal 10 evolves, we will need to keep up with the changes to core. A lot of these changes will be backwards compatible with Drupal 9, so we can safely make them now. At some point I expect we might have to make a new branch to change things that aren't backwards compatible, but that's in the future for now.

This is a meta. No patches here! Open a new issue for each D10 problem found, and add this meta as the parent issue. This is how we will track these issues.

Right now you have to manually trigger a customized D10 test for your patches. I will configure a D10 test in DrupalCI and turn on weekly D10 testing as soon as we get the initial errors fixed in #3255139: [D10] Symfony 6 type hints.