but msgpack-core-0.8.14.jar still built for JVM7 (class version 0.51):

Yes, sbt 1.x can generate JVM7 byte code from Java sources. but sbt 1.x and Scala 2.12 compiler does not run on JVM 7.
This pull request changes scalacOptions not javacOptions. scalacOptions is different from javacOptions.
This change has no effect (just suppress warnings) because Scala 2.12 compiler says follows when set -target:jvm-1.7

[warn] -target is deprecated: -target:jvm-1.7 is deprecated, forcing use of jvm-1.8