Update buttons on New Project > Run CI/CD pipelines for external repositories

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR migrates some of the buttons on the New Project > Run CI/CD pipelines for external repositories page to use Pajamas::ButtonComponent.

Related to &8153

Epic details

There are HAML templates and Ruby files that use plain `%button` tags and/or the `button_tag` helper, and apply some classes to emulate `GlButton`. These should be migrated to `Pajamas::ButtonComponent` instances.

Check Pajamas::ButtonComponent implementation MR. It also has 2 migration examples.

This epic excludes:

1. Dropdown triggers, as they will be handled in &1059.
2. Buttons that look like links, as there's some overlap with &4232.
3. Less easy-to-find buttons which aren't obviously suited to Pajamas::ButtonComponent.
4. Form submit buttons that submit forms built with Rails Form Builder

This epic optionally includes:

1. Links that look like buttons. See the third migration example.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Enterprise Edition required. GitLab team members can request an EE licence following the instructions in developer onboarding.
2. Go to the Run CI/CD for external repository page (Menu > Projects > Create new project > Run CI/CD for external repository)

http://127.0.0.1:3000/projects/new#cicd_for_external_repo

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #364014 (closed)