Set bundle-version suffix of SNAPSHOT versions to the static String "-SNAPSHOT" instead of a timestamp

Despite the discussion in https://www.mail-archive.com/dev@sling.apache.org/msg76177.html we should nowadays switch back the bundle version to having the suffix SNAPSHOT (for reproducible builds to also work on SNAPSHOTs). Compare with https://github.com/bndtools/bnd/issues/3521#issuecomment-572695574 and https://github.com/bndtools/bnd/tree/master/maven/bnd-maven-plugin#reproducible-builds.