Remove support for PHPUnit 5.7

[maxime-rainville]

When we added support for PHP8, we upgraded to PHPUnit 9.5 but retained support for PHPUnit 5.7.

This an ugly hack that should be removed.

Acceptance criteria

• Remove dual implementation of various classes
• Undo the logic to ignore PHPUnit 5.7 tests
• mark the removed API as deprecated in CMS 4
• The hacks are still present in CMS 4
• Breaking changes are reflected in the upgrade docs
• Other than in the upgrade docs, there is no reference to phpunit 5.7

PRs

• API Remove PHPUnit 5.7 compatability hacks #10444
• API Deprecate PHPUnit 5.7 compatability hacks #10448
• DOC Document removing deprecated support for PHPUnit 5.7 developer-docs#63

[sabina-talipova]

Support for PHPUnit 5.7 was removed.
Close.