Refactor runner instructions

What does this MR do and why?

This change refactors the registration_instructions.vue component so it holds more complexity than the admin_register_runner_app.vue component.

This will allow us to enable new group_register_runner_app.vue component more easily.

This is strictly a refactor so no behavior changes are expected

Before

• admin_register_runner_app.vue: Fetches the runner, polls for changes in the registration status, renders instructions. 😩
  • registration_instructions.vue: Displayed the runner registration instructions.

After

• admin_register_runner_app.vue: Renders the instructions. ☺
  • registration_instructions.vue: Fetches the runner, polls for changes in the registration status, displays the runner registration instructions.

Screenshots or screen recordings

See the entire demo at: https://youtu.be/6Bi8XsB73_M

How to set up and validate locally

1. Enable the feature flag: in your console rails c, and then Feature.enable(:create_runner_workflow_for_admin)
2. Use the main version of runner (note that this version has not been released)

$ cd ~
$ curl --output gitlab-runner https://gitlab-runner-downloads.s3.amazonaws.com/main/binaries/gitlab-runner-darwin-amd64
$ chmod +x gitlab-runner
$ ./gitlab-runner install

1. Follow the runner creation instructions from http://gdk.test:3000/admin/runners

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 #383143 (closed)