Remove "certification" assessment attribute from the courses entry if there's a duplicative certification entry for the same content

Why is this change being made?

There are two changes here to learn.yml:

1. Some courses are marked as certifications because there's a certification option available, but we also have a separate entry for the certification itself. This leads to duplicate entries in our certifications lists (e.g., https://about.gitlab.com/learn/certifications/public/ lists both GitLab CI/CD Training and GitLab Certified CI/CD Specialist as two separate entries) - which is confusing. I've marked the course part as Assessment = None to avoid this.
2. I've made all certification links go to the description pages we've created for them (e.g., GitLab Certified Associate goes to https://about.gitlab.com/services/education/gitlab-certified-associate/) instead of directly linking to Edcast. Right now our experience landing in Edcast is suboptimal as per https://gitlab.com/gitlab-com/customer-success/professional-services-group/education-services/-/issues/220 and the services pages we have provide better information about the certification delivery options (instructor led, self service) and course content.

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