Create new https://gitlab.com/gitlab-org/maintainers/gitlab-shell group

Creating a new https://gitlab.com/gitlab-org/maintainers/gitlab-shell group will:

• Make it easier to mention the gitlab-shell maintainers via @gitlab-org/maintainers/gitlab-shell
• Use @gitlab-org/maintainers/gitlab-shell instead of individual users in https://gitlab.com/gitlab-org/gitlab-shell/-/blob/main/.gitlab/CODEOWNERS

Current gitlab-shell maintainers are:

• @ashmckenzie
• @igor.drozdov
• @patrickbajao

cc @sean_carroll