Migrate to Maven 4

[FanJups]

As the Pi4J project uses submodules, it makes a perfect candidate to move to Maven 4. In this new version, submodules got replaced by subprojects, and pom-files in such projects should be simpler to maintain.

Based on this LinkedIn message, the challenge was raised to bump Pi4J to Maven 4...

The develop branch, is the one to work in. Important note: it contains the code for the upcoming V4-release, and has a lot of additional changes compared to the latest release.

This will also need a change in https://github.com/pi4J/pi4j-docker: maybe we should create a V4 Docker here.

[FanJups]

Hello,
We will start working on this issue by February.