Build/Test Tools: Hard deprecate `WP_UnitTestCase_Base::checkRequirements()`.

The `WP_UnitTestCase_Base::checkRequirements()` method calls the `parent::checkRequirements()` method, but this method became `private` in PHPUnit 7.0 via commit  [932238a6a3 sebastianbergmann/phpunit@932238a].

Aside from that, the `TestCase::getAnnotations()` method which is called next is now also removed in PHPUnit 9.5.

WP core does not use the method anymore, and the method only remains to prevent potentially breaking external integration tests relying on the method. However, in effect, the method is not functional anymore in PHPUnit 7.0+.

Follow-up to [893/tests], [894/tests], [896/tests], [918/tests], [30526], [40520], [40564], [43005], [44701], [51559-51577].

Props jrf.
See #46149.
Built from https://develop.svn.wordpress.org/trunk@51602


git-svn-id: http://core.svn.wordpress.org/trunk@51213 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51601';
+$wp_version = '5.9-alpha-51602';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.