6 string references to jsonapi.serializer in Drupal 9

FieldItemNormalizerTest::setUp in core/modules/jsonapi/tests/src/Kernel/Normalizer/FieldItemNormalizerTest.php

LinkCollectionNormalizerTest::setUp in core/modules/jsonapi/tests/src/Kernel/Normalizer/LinkCollectionNormalizerTest.php

RelationshipNormalizerTest::setUp in core/modules/jsonapi/tests/src/Kernel/Normalizer/RelationshipNormalizerTest.php

ResourceObjectNormalizerCacherTest::setUp in core/modules/jsonapi/tests/src/Kernel/EventSubscriber/ResourceObjectNormalizerCacherTest.php

ResourceTestBase::setUp in core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php

SerializerTest::setUp in core/modules/jsonapi/tests/src/Kernel/Serializer/SerializerTest.php