I had to modify several files in order to get rid of a deprecation message:

PHP Deprecated: DI\autowire(): Implicitly marking parameter $className as nullable is deprecated, the explicit nullable type must be used instead in /var/www/vhosts/wordpress/htdocs/umfrage.dgb-bildungswerk.de/vendor/php-di/php-di/src/functions.php on line 48
PHP Deprecated: DI\create(): Implicitly marking parameter $className as nullable is deprecated, the explicit nullable type must be used instead in /var/www/vhosts/wordpress/htdocs/umfrage.dgb-bildungswerk.de/vendor/php-di/php-di/src/functions.php on line 35
PHP Deprecated: GoldSpecDigital\ObjectOrientedOAS\Objects\Schema::boolean(): Implicitly marking parameter $objectId as nullable is deprecated, the explicit nullable type must be used instead in /var/www/vhosts/wordpress/htdocs/umfrage.dgb-bildungswerk.de/vendor/goldspecdigital/oooas/src/Objects/Schema.php on line 232
PHP Deprecated: GoldSpecDigital\ObjectOrientedOAS\Objects\Schema::integer(): Implicitly marking parameter $objectId as nullable is deprecated, the explicit nullable type must be used instead in /var/www/vhosts/wordpress/htdocs/umfrage.dgb-bildungswerk.de/vendor/goldspecdigital/oooas/src/Objects/Schema.php on line 241
PHP Deprecated: GoldSpecDigital\ObjectOrientedOAS\Objects\Schema::number(): Implicitly marking parameter $objectId as nullable is deprecated, the explicit nullable type must be used instead in /var/www/vhosts/wordpress/htdocs/umfrage.dgb-bildungswerk.de/vendor/goldspecdigital/oooas/src/Objects/Schema.php on line 250
PHP Deprecated: GoldSpecDigital\ObjectOrientedOAS\Objects\Schema::object(): Implicitly marking parameter $objectId as nullable is deprecated, the explicit nullable type must be used instead in /var/www/vhosts/wordpress/htdocs/umfrage.dgb-bildungswerk.de/vendor/goldspecdigital/oooas/src/Objects/Schema.php on line 259
PHP Deprecated: GoldSpecDigital\ObjectOrientedOAS\Objects\Schema::string(): Implicitly marking parameter $objectId as nullable is deprecated, the explicit nullable type must be used instead in /var/www/vhosts/wordpress/htdocs/umfrage.dgb-bildungswerk.de/vendor/goldspecdigital/oooas/src/Objects/Schema.php on line 268

./vendor/php-di/php-di/src/Proxy/ProxyFactory.php
43c43
< public function __construct(bool $writeProxiesToFile = false, ?string $proxyDirectory = null)

public function __construct(bool $writeProxiesToFile = false, string $proxyDirectory = null)

./vendor/php-di/php-di/src/Definition/Helper/CreateDefinitionHelper.php
56c56
< public function __construct(?string $className = null)

public function __construct(string $className = null)

./vendor/php-di/php-di/src/Definition/Resolver/ParameterResolver.php
39,40c39,40
< ?MethodInjection $definition = null,
< ?ReflectionMethod $method = null,

    MethodInjection $definition = null,
    ReflectionMethod $method = null,

./vendor/php-di/php-di/src/Definition/Resolver/ObjectCreator.php
212c212
< public static function setPrivatePropertyValue(?string $className, $object, string $propertyName, $propertyValue)

public static function setPrivatePropertyValue(string $className = null, $object, string $propertyName, $propertyValue)

./vendor/php-di/php-di/src/Definition/Source/DefinitionArray.php
39c39
< public function __construct(array $definitions = [], ?Autowiring $autowiring = null)

public function __construct(array $definitions = [], Autowiring $autowiring = null)

./vendor/php-di/php-di/src/Definition/Source/Autowiring.php
23c23
< public function autowire(string $name, ?ObjectDefinition $definition = null);

public function autowire(string $name, ObjectDefinition $definition = null);

./vendor/php-di/php-di/src/Definition/Source/DefinitionNormalizer.php
46c46
< public function normalizeRootDefinition($definition, string $name, ?array $wildcardsReplacements = null) : Definition

public function normalizeRootDefinition($definition, string $name, array $wildcardsReplacements = null) : Definition

./vendor/php-di/php-di/src/Definition/Source/AnnotationBasedAutowiring.php
56c56
< public function autowire(string $name, ?ObjectDefinition $definition = null)

public function autowire(string $name, ObjectDefinition $definition = null)

./vendor/php-di/php-di/src/Definition/ObjectDefinition.php
72c72
< public function __construct(string $name, ?string $className = null)

public function __construct(string $name, string $className = null)

88c88
< public function setClassName(?string $className = null)

public function setClassName(string $className = null)

186c186
< public function setLazy(?bool $lazy = null)

public function setLazy(bool $lazy = null)

./vendor/php-di/php-di/src/Container.php
98,100c98,100
< ?MutableDefinitionSource $definitionSource = null,
< ?ProxyFactory $proxyFactory = null,
< ?ContainerInterface $wrapperContainer = null

    MutableDefinitionSource $definitionSource = null,
    ProxyFactory $proxyFactory = null,
    ContainerInterface $wrapperContainer = null

./vendor/php-di/php-di/src/ContainerBuilder.php
283c283
< public function writeProxiesToFile(bool $writeToFile, ?string $proxyDirectory = null) : self

public function writeProxiesToFile(bool $writeToFile, string $proxyDirectory = null) : self