Pin sbt-jupiter-interface to 0.15.2 for JDK 8 compatibility

0.17+ upgrades to JUnit 6, which requires Java 17; the JDK 8 CI lane failed
compiling msgpack-jackson2 tests against junit-jupiter-api 6.0.3 (class file
version 61). 0.15.2 stays on JUnit 5.14, and the module's explicit
junit-jupiter 5.14.4 dependency wins resolution. Verified by forking the
tests onto a real JDK 8.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014ZZUaHusAjVx6SNu4sA42s

Author: xerial

project/plugins.sbt

@@ -7,7 +7,9 @@ addSbtPlugin("com.github.sbt" % "sbt-osgi"        % "0.11.0-RC1")
 addSbtPlugin("org.scalameta"  % "sbt-scalafmt"    % "2.6.2")
 addSbtPlugin("com.github.sbt" % "sbt-dynver"      % "5.1.1")
 addSbtPlugin("pl.project13.scala" % "sbt-jmh"     % "0.4.8")
-// Runs JUnit 5 (Jupiter) tests from sbt; without this, JUnit 5 tests are silently skipped
-addSbtPlugin("com.github.sbt.junit" % "sbt-jupiter-interface" % "0.19.0")
+// Runs JUnit 5 (Jupiter) tests from sbt; without this, JUnit 5 tests are silently skipped.
+// Pinned to 0.15.x: 0.17+ upgrades to JUnit 6, which requires Java 17 and would break
+// running msgpack-jackson2 tests on JDK 8.
+addSbtPlugin("com.github.sbt.junit" % "sbt-jupiter-interface" % "0.15.2")
 
 scalacOptions ++= Seq("-deprecation", "-feature")