Preselect namespace when buying compute minutes for personal namespace

What does this MR do and why?

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/471681

This makes it so we preselect the namespace in CustomersDot when purchasing compute minutes for a personal namespace from Gitlab.

MR acceptance checklist

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Before	After

How to set up and validate locally

1. Ensure GITLAB_SIMULATE_SAAS = 1 is set
2. Enable migrate_purchase_flows_for_existing_customers feature flag
3. Go to the users User settings -> Usage Quotas -> Pipelines https://staging.gitlab.com/-/profile/usage_quotas
4. Click Buy additional compute minutes