[FLINK-8767] [quickstarts] Set the maven.compiler.source and .target …

…properties for Java Quickstart

Setting these properties helps properly pinning the Java version in IntelliJ.
Without these properties, Java version keeps switching back to 1.5 in some setups.

flink-quickstart/flink-quickstart-java/src/main/resources/archetype-resources/pom.xml

@@ -31,7 +31,10 @@ under the License.
 	<properties>
 		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 		<flink.version>@project.version@</flink.version>
+		<java.version>1.8</java.version>
 		<scala.binary.version>2.11</scala.binary.version>
+		<maven.compiler.source>${java.version}</maven.compiler.source>
+		<maven.compiler.target>${java.version}</maven.compiler.target>
 	</properties>
 
 	<repositories>
@@ -100,8 +103,8 @@ under the License.
 				<artifactId>maven-compiler-plugin</artifactId>
 				<version>3.1</version>
 				<configuration>
-					<source>1.8</source>
-					<target>1.8</target>
+					<source>${java.version}</source>
+					<target>${java.version}</target>
 				</configuration>
 			</plugin>
 
@@ -158,8 +161,8 @@ under the License.
 				<plugin>
 					<artifactId>maven-compiler-plugin</artifactId>
 					<configuration>
-						<source>1.8</source>
-						<target>1.8</target>
+						<source>${java.version}</source>
+						<target>${java.version}</target>
 						<compilerId>jdt</compilerId>
 					</configuration>
 					<dependencies>