chore: migrate phpunit configuration to latest

.gitignore

@@ -1,2 +1,5 @@
 .DS_Store
 vendor
+.phpunit.result.cache
+public/
+composer.lock

composer.json

@@ -44,5 +44,11 @@
         "silverstripe/ldap": "Synchronise extra user attributes from Active Directory with LDAP"
     },
     "minimum-stability": "dev",
-    "prefer-stable": true
+    "prefer-stable": true,
+    "config": {
+        "allow-plugins": {
+            "composer/installers": true,
+            "silverstripe/vendor-plugin": true
+        }
+    }
 }

phpunit.xml.dist

@@ -1,14 +1,14 @@
-<phpunit bootstrap="vendor/silverstripe/framework/tests/bootstrap.php" colors="true">
-    <testsuite name="Default">
-        <directory>tests/</directory>
-    </testsuite>
-
-    <filter>
-        <whitelist addUncoveredFilesFromWhitelist="true">
-            <directory suffix=".php">src/</directory>
-            <exclude>
-                <directory suffix=".php">tests/</directory>
-            </exclude>
-        </whitelist>
-    </filter>
+<?xml version="1.0"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" bootstrap="vendor/silverstripe/framework/tests/bootstrap.php" colors="true" xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.3/phpunit.xsd">
+  <coverage includeUncoveredFiles="true">
+    <include>
+      <directory suffix=".php">src/</directory>
+    </include>
+    <exclude>
+      <directory suffix=".php">tests/</directory>
+    </exclude>
+  </coverage>
+  <testsuite name="Default">
+    <directory>tests/</directory>
+  </testsuite>
 </phpunit>