update code to PHP 7.4 and 8

src/ExternalSerializer/ClosureDeserializer.php

src/ExternalSerializer/ClosureSerializer.php

src/ExternalSerializer/CompositeSerializer.php

src/ExternalSerializer/NullSerializer.php

src/ExternalSerializer/Serializer.php

src/ExternalSerializer/SerializerList.php

src/Mapper/AssertionFailed.php

src/Mapper/CompositeMapper.php

src/Mapper/DebuggingMapper.php

src/Mapper/DeferredAssertionsMapper.php

src/Mapper/MapperList.php

src/Mapper/MatchMapper.php

src/Mapper/PrefixMapper.php

src/Mapper/ReflexivityCheckingMapper.php

src/Mapper/SimpleMapper.php

src/Mapper/SingleNamespaceMapper.php

src/Mapper/TranslatabilityCheckingMapper.php

src/Payload.php

src/PayloadMetadata.php

src/PayloadProcessor.php