Extract subscriptions comparison url to saas.rb

What does this MR do and why?

!118390 (diffs) refactored the feature comparison URL method, which has caused an issue in the JiHu project. Currently, the URL cannot be overridden to point to the JiHu feature comparison page. As a solution, we need to export the feature comparison URL to the lib/gitlab/saas.rb file to enable overriding.

Issues: #409809 (closed) Changelog: changed EE: true

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Related to #409809 (closed)

/cc @chaomao