Make msgpack-java easier to develop and contribute to for Java developers

[odenix]

As a Java developer, I ran into the following issues while working on my first PR (#809):

1. I cloned the repo on Windows. There was no sbt.bat, so I installed sbt manually.
2. The project didn't import correctly into IntelliJ until I realized that I had to install the Scala plugin to get sbt support.
3. When I ran sbt test, I got hundreds of errors.
4. I cloned the repo on WSL/Ubuntu. Now I could run ./sbt test successfully.
5. The WSL/Ubuntu project didn't import correctly into IntelliJ.
   Eventually I found an issue saying that IntelliJ doesn't currently support sbt on WSL.
6. I installed IntelliJ and the IntelliJ Scala plugin on WSL/Ubuntu. Now I could import the project.
7. The project would only compile in IntelliJ after installing JDK 8. Later versions didn't work.
8. sbt and IntelliJ gave many code warnings.
9. Tests for package org.msgpack.value are written in Scala, which was an additional hurdle.
10. I could not run tests with IntelliJ because it doesn't support the Scala test framework used.

Working through the above issues took me about two hours.

Here are some steps that could help to make this project easier to work with for Java developers:

1. Port the few Scala tests to Java/JUnit5.
2. Fix warnings throughout the codebase and/or configure sbt and IntelliJ to suppress them.
3. Raise the minimum supported Java version to 11.
4. Replace sbt with Maven or Gradle.

Does any of this sound attractive? I'd be happy to help.