Revert "[maven-release-plugin] prepare for next development iteration"

This reverts commit 2e060321739a3e12a4db4d1b5b1ef6a60b486f54.
Revert "[maven-release-plugin] prepare release 6.2.1"

This reverts commit b7328d069513c8a4346a52c36f39f35d7bc09100.
Add maven-gpg-plugin
[maven-release-plugin] prepare release 6.2.1
[maven-release-plugin] prepare for next development iteration