Update ChangeLog

Author: sebastianbergmann

ChangeLog-9.0.md

@@ -13,6 +13,7 @@ All notable changes of the PHPUnit 9.0 release series are documented in this fil
 * Implemented [#3746](https://github.com/sebastianbergmann/phpunit/issues/3746): Improve developer experience of global wrapper functions for assertions
 * Implemented [#3914](https://github.com/sebastianbergmann/phpunit/pull/3914): Refactor `PHPUnit\Util\Configuration`
 * Implemented [#4024](https://github.com/sebastianbergmann/phpunit/issues/4024): Make `PHPUnit\TextUI\ResultPrinter` an interface
+* Multiple test case classes (classes that extend `TestCase`) that are declared in a single sourcecode file are no longer supported
 
 ### Removed