Perform code cleanup, drop D8/D9 support, create new 2.1.0 release

Problem/Motivation

The module's code has been maintained for many years by different developers.
Since Drupal 9 reached end of life, only Drupal>=10 should be supported, all the code is written using PHP5/PHP7, it contains a lot of code style errors and deprecations.

A big code cleanup has to be performed, a new Shariff 2.1.0 with Drupal>=10 support, PHP>=8 support should be released afterwards.

Remaining tasks

• Code cleanup/refactor: use PHP 8 syntax
• drop Drupal 8, 9 support
• Fix all open issues
• Make a 2.1.x full release

User interface changes

N/A

API changes

N/A

Data model changes

N/A