Require Java 9 or higher for building

We still support Java 8 at runtime since older launchers shipped with
it, and there are still many installs that use those older launchers.

pom.xml

@@ -152,34 +152,15 @@
 		</dependencies>
 	</dependencyManagement>
 
-	<profiles>
-		<profile>
-			<id>java9</id>
-			<activation>
-				<jdk>[1.9,)</jdk>
-			</activation>
-			<build>
-				<plugins>
-					<plugin>
-						<artifactId>maven-compiler-plugin</artifactId>
-						<configuration>
-							<release>${java.release}</release>
-						</configuration>
-					</plugin>
-				</plugins>
-			</build>
-		</profile>
-	</profiles>
-
 	<build>
 		<plugins>
 			<plugin>
 				<groupId>org.apache.maven.plugins</groupId>
 				<artifactId>maven-compiler-plugin</artifactId>
-				<version>3.6.1</version>
 				<configuration>
 					<source>${java.version}</source>
 					<target>${java.version}</target>
+					<release>${java.release}</release>
 				</configuration>
 			</plugin>
 			<plugin>