GlDisclosureDropdown: Optional max-height prop

After gitlab-org/gitlab!134133 (comment 1611745637) we identified the max-height: none of our GlDisclosureDropdown isn't always appropriate.

Suggested change: Add a new optional prop to the component that can set a max-height.

💡 We probably should keep the current behavior "no max-height" as the default, to not break existing implementations.