test: adjust PHP code to support v8.4

The implicite nullable type has been deprecated.
This fixes it by explicitely typing every nullable type.
PHP is also bumped in composer and drone to v8.4.

tests/acceptance/bootstrap/SettingsContext.php

@@ -350,14 +350,14 @@ class SettingsContext implements Context {
 
 	/**
 	 * @param string $user
-	 * @param array $headers
+	 * @param array|null $headers
 	 *
 	 * @return ResponseInterface
 	 *
 	 * @throws GuzzleException
 	 * @throws Exception
 	 */
-	public function sendRequestGetSettingsValuesList(string $user, array $headers = null): ResponseInterface {
+	public function sendRequestGetSettingsValuesList(string $user, ?array $headers = null): ResponseInterface {
 		return SettingsHelper::getValuesList(
 			$this->featureContext->getBaseUrl(),
 			$user,