Update Ruby from 2.6 to 3.0 on notify in CI

Update Ruby from 2.6 to 3.0 on notify jobs notify_upstream_commit:success and notify_upstream_commit:failure in CI.

In the relevant jobs, we are using gitlab Rubygem, which does not officially support Ruby 3.0 https://github.com/NARKOZ/gitlab/runs/1698033311, but I think we can use it even in Ruby 3.0.

(EDIT (2021-04-19) @tnir:) ^ gitlab Rubygem is now officially supporting Ruby 3.0 https://github.com/NARKOZ/gitlab/runs/1698033311

Filed as NARKOZ/gitlab#611

checklist

• NARKOZ/gitlab#611 was merged