Previously, the custom layout sample could only be built successfully
if spring-boot-dependencies had be installed locally, making its
effective pom available in the local Maven cache.
This commit updates the sample's tests to look at its own pom to
determine the version of Spring Boot that should be used in the
Gradle tests.
Closes gh-8330
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
