[openstack-helm] Dropping train support

Hi all, One of the topics from the latest PTG discussions[0] was looking to drop support of the Train release from our charts due to a lack of interest in maintaining them and several issues with broken images due to dependencies. As a result of those discussions, we'd like to make dropping Train support a goal for this cycle. If anyone is still using Train and has a pressing issue where they cannot upgrade, please reach out and we can discuss a path forward. [0] https://etherpad.opendev.org/p/openstack-helm-zed-ptg Thanks