chore(pom): update release parent version to 3.1
chore(pom): update release parent version to 3.1

- only use nexus-staging-plugin for deploy

Related to SRE-257

ingorichtsmeier/camunda-bpm-platform,camunda/camunda-bpm-platform,langfr/camunda-bpm-platform,ingorichtsmeier/camunda-bpm-platform,langfr/camunda-bpm-platform,langfr/camunda-bpm-platform,langfr/camunda-bpm-platform,ingorichtsmeier/camunda-bpm-platform,ingorichtsmeier/camunda-bpm-platform,camunda/camunda-bpm-platform,camunda/camunda-bpm-platform,ingorichtsmeier/camunda-bpm-platform,ingorichtsmeier/camunda-bpm-platform,langfr/camunda-bpm-platform,camunda/camunda-bpm-platform,camunda/camunda-bpm-platform,langfr/camunda-bpm-platform,camunda/camunda-bpm-platform