Add maven-enforcer-plugin to prevent SNAPSHOT usage in releases

[jmini]

To prevent having SNAPSHOT versions in the dependency tree in released versions, I propose to add the maven-enforcer-plugin to the build with the requireReleaseDeps activated for release build.

Because this project is using SNAPSHOT versions during development, I think that using onlyWhenRelease = true is the appropriate setting for this project.

Documentation: https://maven.apache.org/enforcer/enforcer-rules/requireReleaseDeps.html

Related:

• Release 2.0.10 uses SNAPSHOT dependencies #1053
• prepare release 2.0.3 #829 (comment)

[jmini]

Same change for v1 branch: #1057