Require Drupal 10.3, due to https://www.drupal.org/node/3229001.

Plugins no longer use Doctrine-style annotations in Drupal 11 and 10.3, but PHP attributes. See https://www.drupal.org/node/3229001.

It doesn't make sense for us to write plugins using 10.2-and-earlier style plugin metadata … because they'd trigger deprecation errors on 10.3 and 11.

Furthermore, XB won't be ready for months to come, at which point 10.3 will be readily available.

Logical conclusion: drop 10.2 support.

⚠ This blocks !15 (merged) because that's the first MR to introduce plugin implementations.