Change UuidInterface::getInteger() to always return a string value

src/Uuid.php

@@ -419,7 +419,7 @@ class Uuid implements UuidInterface
      *
      * @inheritDoc
      */
-    public function getInteger()
+    public function getInteger(): string
     {
         return $this->numberConverter->fromHex($this->getHex());
     }

src/UuidInterface.php

@@ -130,10 +130,8 @@ interface UuidInterface extends JsonSerializable, Serializable
 
     /**
      * Returns the 128-bit integer value of the UUID as a string
-     *
-     * @return string
      */
-    public function getInteger();
+    public function getInteger(): string;
 
     /**
      * Returns the least significant 64 bits of the UUID