Default geo-install to SSH and document GITLAB_REPO_URL

What does this merge request do and why?

Unauthenticated HTTPS clones of gitlab-org/gitlab are rejected by Gitaly under server load with a GitLab is currently unable to handle this request due to load error. The geo-install script hardcoded the HTTPS URL with no way to override it in non-interactive mode.

This MR:

1. Changes the default clone URL from HTTPS to SSH (git@gitlab.com:gitlab-org/gitlab.git)
2. Documents the GITLAB_REPO_URL environment variable for overriding the repo URL in non-interactive mode
3. Updates the interactive prompt to recommend SSH and show the HTTPS alternative

Related to !5957 (merged)

How to set up and validate locally

1. Review the script changes in support/geo-install.
2. Run the script interactively to confirm the new prompts display correctly.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This MR references an issue describing the change.
• This change is backward compatible. If not, include steps to communicate to users.
• Tests added for new functionality. If not, raise an issue to follow-up.
• Observability added/updated (logging, metrics, tracing).
• Documentation added/updated.
• Announcement added for notable changes.
• gdk doctor test added.