Use JavaVersion to specify java version

Server-Protocol-Buffer/build.gradle

@@ -5,8 +5,8 @@ sourceSets {
     main.java.srcDirs += "${protobuf.generatedFilesBaseDir}/main/javalite"
 }
 
-sourceCompatibility = "8"
-targetCompatibility = "8"
+sourceCompatibility = JavaVersion.VERSION_1_8
+targetCompatibility = JavaVersion.VERSION_1_8
 
 dependencies {
     implementation fileTree(dir: 'libs', include: ['*.jar'])