Drupal 12 + PHP Compatibility

Problem/Motivation

There are several 'compatibility' issues that intertwine with each other. Typically these issues will be merged as part of the next Major Drupal compatibility release.

However, since there aren't issues by the bot made yet, I'm adding it here and we can link the sub-issues below.

This issue combines the following issues

• #3587982: Modernize tests for PHPUnit 12 compatibility
• #3556305: Convert Purge plugins to attributes
• #3447548: Clean up services, add autowiring, make use of new features, and fix deprecations
• #3511668: PHP 8.4 support implicit null deprecated
• #3564459: Drupal 11.3 compatibility: Not supporting attribute discovery in Drupal\purge\Plugin\Purge\Processor\PluginManager
• #3139376: Replace assertions involving calls to is_object() with assertIsObject()/assertIsNotObject()
• #3139358: Replace assertions involving calls to is_array() with assertIsArray()/assertIsNotArray()
• #3295008: Update project page
• #3264141: [REGRESSION] Purge 8.x-3.1+ incompatible with Config Split

Remaining tasks

* Merge in all the issues
* Re-run / update tests
* Ensure Drupal 10 compatibility remains
* Drop Drupal 9 compatibility (D9 usage is that of D8 when it was dropped for D11)