Remove customers portal /plans page and replace it with https://customers.gitlab.com/

Why is this change being made?

We are migrating away from https://customers.gitlab.com/plans Home page will be https://customers.gitlab.com/ and is going to show active subscription (empty if none purchased). /plans page will removed completely.

Note: there are more /plans occurrences, they are going to be handled separately

Related to gitlab-org/customers-gitlab-com#3645 (closed)

Author Checklist

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• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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