Overview of feature request

Islandora code needs to work on Drupal 10 before the EOL of Drupal 9, which is fast approaching.

This is a meta-issue to link together work done in various repositories.

Basic steps that I've found useful (pls add/edit):

• Run Drupal Rector on the module (on D9) to fix known deprecated code.
• Run PHPUnit locally, and take note of deprecation notices (that aren't errors). This sometimes finds deprecations that Rector misses.
• Install Upgrade Status and scan the code. Fix outstanding issues.
• Declare the module compatible with Drupal 10. Install it on D10 and test that it still functions.
• Fix problems that come up in the code in D10 that weren't caught by Rector or the other tools
• Test the module again with Drupal 9 to make sure it still works. When there are incompatibilities in code (e.g. parent class signatures change) that mean a single codebase can't satisfy D9 and D10, we'll have to make a new major version branch for the Drupal 10 version.
• Celebrate!