Use Implementation-Build rather than Plugin-GitHash

[basil]

See jenkinsci/pom#419 and jenkinsci/maven-hpi-plugin#471. In jenkinsci/pom#419 I am planning to add the default specification and default implementation entries in core and core component .jar and .war files, consistent with plugin .jar and .hpi files. In jenkinsci/pom#419 I am also planning to add the Git hash as Implementation-Build to core and core component .jar and .war files, and in jenkinsci/maven-hpi-plugin#471 I am renaming this information from Plugin-GitHash (recently added) to Implementation-Build for consistency. This PR adapts to jenkinsci/maven-hpi-plugin#471. While I am temporarily supporting Plugin-GitHash for now, I plan to rip out Plugin-GitHash shortly (in a month or so) once it's no longer used by anything (not much is using it today, just the AWS Java SDK plugin I think).