Define specific support expectations at each tier

Before merging gitlab-com/www-gitlab-com!16451 (merged), @tcooney mentioned that we may want to "clarify support expectations across each GitLab product solution/package available for purchase inside the definition of each package."

In other words, don't define "Premium Support" outside of tiers and selectively apply it to certain plans; instead, we should look at each one of our plans and create specific support definitions within each package.

We should discuss what these new support expectations may look like and open MRs to make these changes.