Redirect CustomersDot /plans page to pricing

What does this MR do and why?

We are migrating from https://customers.gitlab.com/plans to https://about.gitlab.com/pricing/ and need to update links to /plans page that present in GitLab.

Related to customers-gitlab-com#3751 (closed)

Screenshots or screen recordings

Screenshot	Before	After
	https://customers.gitlab.com/plans	leads to https://about.gitlab.com/pricing/
	https://customers.gitlab.com/plans	leads to https://about.gitlab.com/pricing/
	https://customers.gitlab.com/plans	leads to https://about.gitlab.com/pricing/
	https://customers.gitlab.com/plans	leads to https://about.gitlab.com/pricing/

How to set up and validate locally

1. Enable redirect_plans_page flag

   Feature.enable(:redirect_plans_page)

Case 1

1. Login as a user with a free account
2. Open dropdown menu
3. Check that the Upgrade button leads to https://about.gitlab.com/pricing/ page

Case 2

1. Login as a user with a free account
2. Navigate to an issue in any project
3. Click Learn more in the sidebar next to the Weight feature (by default this feature is disabled for free accounts)
4. Check that the Premium plan leads to https://about.gitlab.com/pricing/ page

Case 3

1. Login as an admin with an expired license
2. Navigate to an issue in any project
3. Click Learn more in the sidebar next to the Weight feature
4. Check that the Buy EE button leads to https://about.gitlab.com/pricing/ page

Case 4

1. Login as an admin and make sure that no license is applied
2. Navigate /admin/subscription
3. Check the button Buy subscription in the Subscription box leads to https://about.gitlab.com/pricing/ page

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.