Replace fa-chevron-down icons in clusters/gcp/form

There's an ongoing effort to replace remaining Fontawesome icons with our GitLab SVGs.

The following icon instances/references in the clusters gcp form should be replaced:

• https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/views/clusters/clusters/gcp/_form.html.haml#L35
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/views/clusters/clusters/gcp/_form.html.haml#L46
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/views/clusters/clusters/gcp/_form.html.haml#L62

Which icon should we use instead?

Fontawesome icon (old)	GitLab SVG (new)
chevron-down	chevron-down