Upgrade to use Java5 annotations

See https://maven.apache.org/plugin-tools/maven-plugin-tools-annotations/index.html
and https://maven.apache.org/plugin-developers/cookbook/plexus-plugin-upgrade.html.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
wip

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>