Promote plugin dependency management

This commit promotes the plugin dependency management for the
`maven-enforcer-plugin` and `maven-invoker-plugin` to the root. That way
these can be used in samples as well rather than having a separate copy.

Closes gh-7517

spring-boot-samples/pom.xml

@@ -153,7 +153,6 @@
 			<plugin>
 				<groupId>org.apache.maven.plugins</groupId>
 				<artifactId>maven-enforcer-plugin</artifactId>
-				<version>1.4</version>
 				<executions>
 					<execution>
 						<id>enforce-rules</id>