Declare which drupal core versions support in composer.json

[andybroomfield]

Since the Drupal 10.1 was set as the minimum in #190 tests that still use Drupal 9 are failing as they cannot install core.
This happened as localgov_core is still treated as valid.
In Alert banner we declare which version of Drupal core we support so that the right version is downloaded with composer.

We should do that here, and consider a retrospective release of localgov_core 2.12 that declares ^9||^10 so the right versions get installed? (Or maybe it is too late....)