PHPUnit 9.0: Replace references to expectExceptionMessageRegExp() w…

…ith `expectExceptionMessageMatches()`

Based on the changelog of PHPUnit 9.0, the `expectExceptionMessageRegExp()` method has been removed and `expectExceptionMessageMatches()` should be used instead.

Refs:
* https://github.com/sebastianbergmann/phpunit/blob/9.0.1/ChangeLog-9.0.md
* sebastianbergmann/phpunit#3957

src/writing-tests-for-phpunit.rst

@@ -651,7 +651,7 @@ whether an exception is thrown by the code under test.
 In addition to the ``expectException()`` method the
 ``expectExceptionCode()``,
 ``expectExceptionMessage()``, and
-``expectExceptionMessageRegExp()`` methods exist to set up
+``expectExceptionMessageMatches()`` methods exist to set up
 expectations for exceptions raised by the code under test.
 
 .. admonition:: Note