Issue a warning when using ServiceBuilder in ext plugin

• Type: Task
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 6.1.10 EE1, 6.1.0 GA1
• Fix Version/s: 6.2.0 GA1, 6.1.1 GA2, 6.1.20 EE GA2
• Component/s: Plugins
• Labels: None
• Git Pull Request: https://github.com/brianchandotcom/liferay-maven-support/pull/18

ServiceBuilderMojo should issue a warning similarly to plugins sdk that service builder will be deprecated in ext plugins at some point.