End compatibility with Drupal 8

Problem/Motivation

Drupal 8 has been officially unsupported since November 2021 → but the Scheduler module has so far continued to be compatible with Drupal 8. However, this needs to change because we can no longer support PHP7.4, due to 📌 Fix @Constraint annotation for plugin Active where the attributes syntax needs PHP8.

Drupal 8 does not run on PHP8 - see this failed Drupal 8 Composer job where the PHP version was set to 8.0

Your requirements could not be resolved to an installable set of packages.
  Problem 1
    - Root composer.json requires drupal/core-recommended ^8.9 -> satisfiable by drupal/core-recommended[8.9.0-beta1, ..., 8.9.x-dev].
    - composer/installers v1.7.0 requires composer-plugin-api ^1.0 -> found composer-plugin-api[2.6.0] but it does not match the constraint.
    - composer/semver[1.1.0, ..., 1.7.1] require php ^5.3.2 || ^7.0 -> your php version (8.0.28) does not satisfy that requirement.
    - drupal/core 8.9.20 requires php ^7.0.8 -> your php version (8.0.28) does not satisfy that requirement.
    - drupal/core-recommended[8.9.0-beta1, ..., 8.9.0-rc1] require composer/installers v1.7.0 -> satisfiable by composer/installers[v1.7.0].
    - drupal/core-recommended[8.9.0, ..., 8.9.x-dev] require composer/semver 1.5.1 -> satisfiable by composer/semver[1.5.1].
    - drupal/core-recommended 8.9.20 requires drupal/core 8.9.20 -> satisfiable by drupal/core[8.9.20].

Proposed resolution

The existing latest releases 8.x-1.5 → and 2.2.1 → will be the last to be compatible with Drupal 8

Remaining tasks

User interface changes

API changes

Data model changes