Update plugins, including MiMa

For Spark 3, we should update plugins to their latest version where possible, to pick up miscellaneous fixes. In particular we can update MiMa to 0.3.0, though that introduces some new errors on old changes due to some changes in MiMa.

Most SBT plugins can't really be updated further without updating to SBT 1.x, and that will require some changes to the build, and it generally seems like all of these new versions are for Scala 2.12+, including the new zinc. That will probably be a bigger change but only after deciding to drop Scala 2.11 support.