PHP8.4 deprecations

[kynx]

Assortment from my unit tests on PHP8.4 (there's a bunch more caused by symfony/yaml version, but that's another matter):

1) /github/workspace/vendor/devizzent/cebe-php-openapi/src/SpecBaseObject.php:404
cebe\openapi\SpecBaseObject::resolveReferences(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead

2) /github/workspace/vendor/devizzent/cebe-php-openapi/src/SpecObjectInterface.php:43
cebe\openapi\SpecObjectInterface::resolveReferences(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead

3) /github/workspace/vendor/devizzent/cebe-php-openapi/src/spec/Paths.php:249
cebe\openapi\spec\Paths::resolveReferences(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead

11) /github/workspace/vendor/devizzent/cebe-php-openapi/src/spec/PathItem.php:153
cebe\openapi\spec\PathItem::resolveReferences(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead

12) /github/workspace/vendor/devizzent/cebe-php-openapi/src/spec/Responses.php:239
cebe\openapi\spec\Responses::resolveReferences(): Implicitly marking parameter $context as nullable is deprecated, the explicit nullable type must be used instead

[kynx]

Gah. Looked again and saw these were fixed a year ago. The problem was that I was pulling in this via league/openapi-psr7-validator which has a shonky lower version constraint 🤦 Sorry for the noise.