Drop support for Drupal 8 and 9, require PHP 8.1

Problem/Motivation

Drupal 10 is now the lowest supported version. If we drop support for Drupal 8 and 9 we can set the minimum required PHP version to 8.1.

This opens the door to adopting the improvements brought by PHP 8, better type support, annotations etc.

Proposed resolution

• Drop support for Drupal 8 + 9.
• Require PHP 8.1.
• Announce this in mailchimp.info.yml and composer.json.
• Update automated test configuration to only run tests on supported versions.

Depending on whether backwards compatibility support is considered important by the maintainers we can do this in a minor 2.3.0 release or major 3.0.0 release