downgrade MiMa to fix 2.13.4 upgrade

MiMa 0.8.0 introduced a regression: mimaPreviousClassfiles fails,
trying to resolve previous, released artifacts with 2.13.4 despite
explicitly setting 2.13.3 in mimaPreviousArtifacts.

project/plugins.sbt

@@ -3,7 +3,7 @@ addSbtPlugin("com.eed3si9n" % "sbt-buildinfo" % "0.10.0")
 addSbtPlugin("com.typesafe.sbt" % "sbt-ghpages" % "0.6.3")
 addSbtPlugin("com.typesafe.sbt" % "sbt-site" % "1.4.1")
 addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "0.15.0")
-addSbtPlugin("com.typesafe" % "sbt-mima-plugin" % "0.8.1")
+addSbtPlugin("com.typesafe" % "sbt-mima-plugin" % "0.7.0")
 addSbtPlugin("com.47deg" % "sbt-microsites" % "0.7.27")
 addSbtPlugin("com.eed3si9n" % "sbt-unidoc" % "0.4.3")
 addSbtPlugin("org.scoverage" % "sbt-scoverage" % "1.6.1")