Transferring a public group to a private group doesn't remove code from the Elastichsearch API search result

### Summary When a public group with public projects is transferred to a private group, the code and the wiki of the public project, although now should be private, it is still reachable through search APIs. I set the severity as "medium" and not "high", because any new action over the project issues a re indexing (or some actions, not sure), so if the transfer is for "archiving" purposes it is a problem, but if after the transfer other activities happen, then it is not a problem, cause the project will be removed from the index. ### Steps to reproduce Alice creates the public group "Example", and a public project named "Example-project" inside the group. In the readme of the project, Alice writes "Example". Now, Alice creates a private group called "private", and transfer all the "Example" group to the "private" group. If Bob (totally unrelated to Alice), search for "Example" instance-wide, will not find anything on the interface, but the count of the results will be "1" (see screenshot). If he uses the APIs (e.g. http://localhost/api/v4/search?search=password&scope=blobs), he will receive the results back with the information that should be private. This happens also with wiki_blobs. This doesn't happen transferring single projects, but only transferring entire groups ### Output of checks #### Results of GitLab environment info ``` System information System: Proxy: no Current User: git Using RVM: no Ruby Version: 2.6.3p62 Gem Version: 2.7.9 Bundler Version:1.17.3 Rake Version: 12.3.3 Redis Version: 3.2.12 Git Version: 2.22.0 Sidekiq Version:5.2.7 Go Version: unknown GitLab information Version: 12.5.2-ee Revision: c1b3929bc67 Directory: /opt/gitlab/embedded/service/gitlab-rails DB Adapter: PostgreSQL DB Version: 10.9 URL: http://aldebaran HTTP Clone URL: http://aldebaran/some-group/some-project.git SSH Clone URL: git@aldebaran:some-group/some-project.git Elasticsearch: yes Geo: no Using LDAP: no Using Omniauth: yes Omniauth Providers: GitLab Shell Version: 10.2.0 Repository storage paths: - default: /var/opt/gitlab/git-data/repositories GitLab Shell path: /opt/gitlab/embedded/service/gitlab-shell Git: /opt/gitlab/embedded/bin/git ``` ## Impact Alice thinks her code is now private, but it is not, unless she continues working on the project