Use Gradle Toolchain in build

Closes gh-442

build.gradle

@@ -33,8 +33,9 @@ configure(moduleProjects) {
 	apply plugin: 'org.springframework.graphql.compiler'
 
 	java {
-		sourceCompatibility = JavaVersion.VERSION_1_8
-		targetCompatibility = JavaVersion.VERSION_1_8
+		toolchain {
+			languageVersion = JavaLanguageVersion.of(8)
+		}
 	}
 
 	pluginManager.withPlugin("kotlin") {

buildSrc/build.gradle

@@ -9,9 +9,6 @@ repositories {
 	maven { url "https://repo.spring.io/release" }
 }
 
-sourceCompatibility = 1.8
-targetCompatibility = 1.8
-
 dependencies {
 	checkstyle "io.spring.javaformat:spring-javaformat-checkstyle:${javaFormatVersion}"
 	implementation("io.spring.javaformat:spring-javaformat-gradle-plugin:${javaFormatVersion}")