Merge gl-button with gl-new-button and make sure gitlab is updated with the new button component that conforms to the design system

This issue describes a plan to deprecate the obsolete <gl-button> implementation and migrate all instances of that component to the new <gl-button> implementation.

Approach (preferred): Organize a button and dropdown migration effort

Goals

• Merge button and dropdown components without introducing new abstractions.
• Migrate all <gl-button> and <gl-dropdown> instances incrementally.

Why

• It is a boring and predictable solution.
• We can break down the effort into multiple developers.
• It is a proven approach. It worked pretty well with the BootstrapVue upgrade. It’s also worked well with the Jestsodus and lodash projects.

How

• Rename the current GlButton component to something like GlDeprecatedButton. !1223 (merged)
• When the change is released, migrate GlButton to GlDeprecatedButton in consuming projects:
  • GitLab: gitlab!27835 (merged)
  • Customer Portal: https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/1189
  • Status Page: N/A (not using deprecated button component)
• Rename GlNewButton to GlButton. !1291 (merged)
• Update all GitLab UI components that rely on either GlButton or GlNewButton by replacing instances of <gl-button to <gl-deprecated-button and <gl-new-button to <gl-button. !1291 (merged)
• When the change is released, migrate GlNewButton to GlButton in consuming projects:
  • GitLab: gitlab!29343 (merged)
  • Customer Portal: https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/1238
  • Status Page: status-page!67 (merged)
• Document all <gl-deprecated-button> instances and how to replace them with the new component. For example:

#### `app/assets/javascripts/monitoring/components/dashboard.vue`

- Replace `<gl-deprecated-button variant="primary" />` with `<gl-button />`
- Replace `<gl-deprecated-button variant="outline-success" />` with `<gl-button variant="success" category="secondary" />`

• Indicate that each file should be migrated in a separate Merge Request. The Merge Request should contain before/after screenshots of each button migration for easier UX review.
• Create an issue for each file and collect those issues in an epic. Creating these issues is something we can likely automate.
• Request in the #frontend channel, help on closing those issues.
• Once all the issues are closed, delete the <gl-deprecated-button> component. !1908 (merged)

Follow the same strategy with the dropdown component.

Good to have

• Provide a list of codemod expressions to facilitate the migration effort.
• Implement an ESLint rule that warns about the usage of the deprecated button component in a Vue file.

Done ✨