Bump sbt-jmh to 0.4.8 for sbt 2 compatibility

main migrated the build to sbt 2 (feb87ef) without sbt-jmh, since
sbt-jmh 0.4.7 has no sbt-2-compatible artifact. This branch's
msgpack-jackson3 JMH benchmarks depend on sbt-jmh, so after merging
main the plugin fails to resolve. 0.4.8 publishes an sbt_2_3 build.

Author: komamitsu

project/plugins.sbt

@@ -6,6 +6,6 @@ addSbtPlugin("org.xerial.sbt" % "sbt-jcheckstyle" % "0.3.0")
 addSbtPlugin("com.github.sbt" % "sbt-osgi"        % "0.11.0-RC1")
 addSbtPlugin("org.scalameta"  % "sbt-scalafmt"    % "2.6.1")
 addSbtPlugin("com.github.sbt" % "sbt-dynver"      % "5.1.1")
-addSbtPlugin("pl.project13.scala" % "sbt-jmh"     % "0.4.7")
+addSbtPlugin("pl.project13.scala" % "sbt-jmh"     % "0.4.8")
 
 scalacOptions ++= Seq("-deprecation", "-feature")