Add explicit dependency on Gson

It used to be pulled in transitively by selenium-api but that's no
longer that case.

See gh-8381

spring-boot-test-autoconfigure/pom.xml

@@ -44,6 +44,11 @@
 			<artifactId>jackson-databind</artifactId>
 			<optional>true</optional>
 		</dependency>
+		<dependency>
+			<groupId>com.google.code.gson</groupId>
+			<artifactId>gson</artifactId>
+			<optional>true</optional>
+		</dependency>
 		<dependency>
 			<groupId>com.jayway.jsonpath</groupId>
 			<artifactId>json-path</artifactId>