Make mmtc-core jar multi-release

[rmseng]

Make the mmtc-core jar a multi-release jar so it can serve as a dependency to more modern Java builds, via e.g. the following:

tasks.jar {
    manifest {
        attributes(
            "Implementation-Title" to project.name,
            "Implementation-Version" to project.version,
            "Multi-Release" to "true"
        )
    }
}