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/spring-boot-sample-custom-layout/pom.xml

@@ -41,7 +41,6 @@
 			<plugin>
 				<groupId>org.apache.maven.plugins</groupId>
 				<artifactId>maven-invoker-plugin</artifactId>
-				<version>1.10</version>
 				<configuration>
 					<cloneProjectsTo>${project.build.directory}/it</cloneProjectsTo>
 					<settingsFile>src/it/settings.xml</settingsFile>