Refactor to extract instructions modal to a separate component in Google Cloud runner registration

Extract the modal in ~/ci/runner/components/registration/google_cloud_registration_instructions.vue to another component.

The following discussion from !146545 (merged) should be addressed:

@mrincon started a discussion:

Note: Testing forms with shallowMount is difficult. Instead, I used mount to render the entire forms components and use "real" DOM events. As a must test the interaction with the modal, this helps test our gl-modal when using mount.

I'll open a follow up to extract the modal to another component to the "unit" is not so large and we can test using the best mounting depending on the situation.