Fix compatiblity of methods with native return type in PHP 8.1

src/Error/Error.php

@@ -9,6 +9,7 @@
 use GraphQL\Language\Source;
 use GraphQL\Language\SourceLocation;
 use JsonSerializable;
+use ReturnTypeWillChange;
 use Throwable;
 use Traversable;
 
@@ -299,6 +300,7 @@ public function getExtensions(): ?array
      * @return array<string, mixed> data which can be serialized by <b>json_encode</b>,
      * which is a value of any type other than a resource.
      */
+    #[ReturnTypeWillChange]
     public function jsonSerialize(): array
     {
         return FormattedError::createFromException($this);

src/Language/SourceLocation.php

@@ -5,6 +5,7 @@
 namespace GraphQL\Language;
 
 use JsonSerializable;
+use ReturnTypeWillChange;
 
 class SourceLocation implements JsonSerializable
 {
@@ -40,6 +41,7 @@ public function toSerializableArray(): array
     /**
      * @return array{line: int, column: int}
      */
+    #[ReturnTypeWillChange]
     public function jsonSerialize(): array
     {
         return $this->toArray();

src/Type/Definition/Type.php

@@ -13,6 +13,7 @@
 use GraphQL\Utils\Utils;
 use JsonSerializable;
 use ReflectionClass;
+use ReturnTypeWillChange;
 
 use function array_keys;
 use function array_merge;
@@ -287,6 +288,7 @@ public function assertValid(): void
         Utils::assertValidName($this->name);
     }
 
+    #[ReturnTypeWillChange]
     public function jsonSerialize(): string
     {
         return $this->toString();