Migrate phpunit.xml schema as per suggestion in its output ...

$ phpunit

PHPUnit 9.3.7 by Sebastian Bergmann and contributors.

Warning:       Your XML configuration validates against a deprecated schema.
Suggestion:    Migrate your XML configuration using "--migrate-configuration"!

Also, removed code coverage whitelist since it's not used and misconfigured anyway.

phpunit.xml.dist

@@ -1,5 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<phpunit backupGlobals="false"
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.3/phpunit.xsd"
+         backupGlobals="false"
          backupStaticAttributes="false"
          bootstrap="vendor/autoload.php"
          colors="true"
@@ -13,11 +15,6 @@
             <directory suffix="Test.php">./tests</directory>
         </testsuite>
     </testsuites>
-    <filter>
-        <whitelist processUncoveredFilesFromWhitelist="true">
-            <directory suffix=".php">./app</directory>
-        </whitelist>
-    </filter>
     <php>
         <env name="APP_ENV" value="testing"/>
         <env name="CACHE_DRIVER" value="array"/>