Issue #3603786: Raise minimum core versions and fix coding standards flagged on current core
Set the minimum core to ^10.6 || ^11.3 || ^12 in both the info file and composer.json.
Bring the module back in line with core's phpcs.xml.dist, which has moved ahead since the last green pipeline:
- Add declare(strict_types=1) to the Kernel and Functional test classes; core enforces it under /tests/.
- Correct {@inheritDoc} to {@inheritdoc} in the two CKEditor plugins.
Closes #3603786