Adds Mobile Devops to codeowners rake task

What does this MR do and why?

I just worked on a merge request for groupmobile devops, and realized they weren't in the codeowners.rake list. They're a single-engineer group. It's visible in data/stages.yml: https://gitlab.com/gitlab-com/www-gitlab-com/-/blob/master/data/stages.yml?ref_type=heads#L2804

I checked the Technical Writing project (https://gitlab.com/gitlab-org/technical-writing/-/issues/) and didn't find any mentions of this group. Figured I'd create this MR now, because I won't remember after the long weekend.

(If we don't list single-engineer groups in the rake task file, we should probably add a code comment explaining so.)

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