minor #19183 Update Create Framework / Unit Testing page (faissaloux)

This PR was squashed before being merged into the 5.4 branch.

Discussion
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Update `Create Framework / Unit Testing` page

Hello 👋🏻

In this PR I have updated the deprecated `will($this->returnValue())` to `willReturn()`, and the phpunit config file.

Commits
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97bfcfc Update `Create Framework / Unit Testing` page

Author: javiereguiluz

create_framework/unit_testing.rst

@@ -12,7 +12,7 @@ using `PHPUnit`_. At first, install PHPUnit as a development dependency:
 
 .. code-block:: terminal
 
-    $ composer require --dev phpunit/phpunit
+    $ composer require --dev phpunit/phpunit:^9.6
 
 Then, create a PHPUnit configuration file in ``example.com/phpunit.xml.dist``:
 
@@ -21,7 +21,7 @@ Then, create a PHPUnit configuration file in ``example.com/phpunit.xml.dist``:
     <?xml version="1.0" encoding="UTF-8"?>
     <phpunit
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-        xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.3/phpunit.xsd"
+        xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.6/phpunit.xsd"
         backupGlobals="false"
         colors="true"
         bootstrap="vendor/autoload.php"