Change the steps of specifying a dev as a code owner in Trainee database maintainer template

Since it's possible to specify groups in CODEOWNERS https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14071, adding a developer to @gl-database becomes a sufficient step to specify the developer as a codeowner: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/.gitlab/CODEOWNERS

It would be cool to change the template accordingly: https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/.gitlab/issue_templates/Trainee%20database%20maintainer.md

For example, https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/.gitlab/issue_templates/Trainee%20backend%20maintainer.md is already changed this way by !28148 (merged)

Closes #5065 (closed)