Migrate button-class-vue in todo.vue

What does this MR do and why?

Detected usage of gl-button class in app/assets/javascripts/sidebar/components/todo_toggle/todo.vue. It is expected to use GlButton component from @gitlab/ui.

It's a part of Pajamas adoption project

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Open some project.
2. Goto Monitor > Alerts in your project’s sidebar.
3. Create new alert from the rails console (please update project id):

AlertManagement::Alert.create!(title: 'Test alert', project: Project.find(2), started_at: Time.now)

4. Open alert.
5. Verify Todo button in the right sidebar.

Related to #418249 (closed)