minor #21268 [Testing] PHPUnit Version update (Raistlfiren)

This PR was submitted for the 7.3 branch but it was merged into the 6.4 branch instead.

Discussion
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[Testing] PHPUnit Version update

PHPUnit V10 uses `bootstrap` instead of `extension`.

- Element 'extension': This element is not expected. Expected is ( bootstrap ).

Commits
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c0a518d PHPUnit Version update

Author: javiereguiluz

testing.rst

@@ -397,6 +397,8 @@ Now, enable it as a PHPUnit extension:
 
         <extensions>
             <extension class="DAMA\DoctrineTestBundle\PHPUnit\PHPUnitExtension"/>
+            <!-- Greater than PHPunit v10 -->
+            <bootstrap class="DAMA\DoctrineTestBundle\PHPUnit\PHPUnitExtension"/>
         </extensions>
     </phpunit>