[tc][networking-l2gw] Finish the retirement for networking-l2gw and networking-l2gw-tempest-plugin

Hello Everyone, networking-l2gw and networking-l2gw-tempest-plugin projects were retired in Ocata[1] but their retirement was not completed. I have started the retirement completion as discussed in TC and neutron team. - https://review.opendev.org/#/q/topic:complete-retirement-networking-l2gw+(st...) Please let me know any injection to that. Their development can be continued under opendev as mentioned in this doc - https://governance.openstack.org/tc/reference/dropping-projects.html#continu... [1] https://review.opendev.org/#/c/392010/2 -gmann