Revert "Allow maven build with JDK 1.8"

Revert commit fd15b62a9d to
ensure that we don't accidentally use Java 8 features.

spring-boot-parent/pom.xml

@@ -263,7 +263,7 @@
 											[1.3.1,)
 										</versionRange>
 										<goals>
-											<goal>enforce</goal>
+											<goal>enforce-once</goal>
 										</goals>
 									</pluginExecutionFilter>
 									<action>
@@ -325,12 +325,12 @@
 					<execution>
 						<id>enforce-rules</id>
 						<goals>
-							<goal>enforce</goal>
+							<goal>enforce-once</goal>
 						</goals>
 						<configuration>
 							<rules>
 								<requireJavaVersion>
-									<version>(1.7,1.8]</version>
+									<version>(1.7,1.8)</version>
 								</requireJavaVersion>
 							</rules>
 						</configuration>