Update plugin to latest upstream release

[dotta]

I had an exchange with a prospect who is using the latest (4.3.1) scala-maven-plugin and downgrading to 3.2.2 is too much effort (e.g., incremental compilation doesn't work when using Java9+ - davidB#217). We agreed I'd reach backthe moment we have an updated release.

[dotta]

And it turns out that the scala-maven-plugin 3.2.2 is incompatible with maven-compiler-plugin 3.1+ if a project has mixed Scala and Java sources. In this scenario, no binaries for the Java sources are produced. The workaround is to downgrade the version of the maven-compiler-plugin to 3.0 (or older). This can be seen in practice here: https://github.com/dotta/scala-maven-example