Use Maven toolchain for Surefire plugin

gson/pom.xml

@@ -71,13 +71,14 @@
         <artifactId>maven-surefire-plugin</artifactId>
         <version>3.0.0-M5</version>
         <configuration>
+          <jdkToolchain>
+            <!-- Require at least Java 9 for tests, see below -->
+            <version>[9,)</version>
+          </jdkToolchain>
           <!-- Deny illegal access, this is required for ReflectionAccessTest -->
           <!-- Requires Java >= 9; Important: In case future Java versions 
             don't support this flag anymore, don't remove it unless CI also runs with 
-            that Java version. Ideally would use toolchain to specify that this should 
-            run with e.g. Java 11, but Maven toolchain requirements (unlike Gradle ones) 
-            don't seem to be portable (every developer would have to set up toolchain 
-            configuration locally). -->
+            that Java version -->
           <argLine>--illegal-access=deny</argLine>
         </configuration>
       </plugin>