Add viktomas as gitlab-cli maintainer

Why is this change being made?

When we adopted the GitLab CLI project, I helped out with the initial housekeeping, and I had to have maintainer privileges for that.

I'm happy to stay as an interim maintainer, especially because I know how it is when there is only one maintainer on a project (VS Code Extension). I would like @garyh to be able to take longer leave 😉

Unless the CLI work starts overlapping with the GitLab Remote Development I would like to step down eventually. But in the meantime, I'm happy to help.

/cc @oregand

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