fix(dataproducer): Fix language definition to be single value in entity reference producers (drupal-graphql#1241)

Author: chrfritsch

src/Plugin/GraphQL/DataProducer/Field/EntityReferenceLayoutRevisions.php

@@ -35,7 +35,6 @@
  *     ),
  *     "language" = @ContextDefinition("string",
  *       label = @Translation("Language"),
- *       multiple = TRUE,
  *       required = FALSE
  *     ),
  *     "bundle" = @ContextDefinition("string",

src/Plugin/GraphQL/DataProducer/Field/EntityReferenceRevisions.php

@@ -35,7 +35,6 @@
  *     ),
  *     "language" = @ContextDefinition("string",
  *       label = @Translation("Language"),
- *       multiple = TRUE,
  *       required = FALSE
  *     ),
  *     "bundle" = @ContextDefinition("string",