Replace migration guide URL with the correct one

[samnajian]

Description

This PR updates the link to the migration guide.
Fixes WOOSHIP-1511

Steps to test:

• Make a build from this branch.
• Upload to your test site.
• Force the survey banner by using &force_survey=1 in the order page URL where you want to make a label purchase.
• Make a label purchase, fillout the survey form, submit and on the success screen verify the the link to the guides page is this: https://woocommerce.com/document/woocommerce-shipping-and-tax/woocommerce-shipping/#how-do-i-migrate-from-wcst and verify the link text doesn't contain step-by-step.

[ayushpahwa]

Apologies for the delay in review. Code LGTM and the testing instructions work as described.

[ayushpahwa]

The changelog seems to be missing, is that expected?

[samnajian]

The changelog seems to be missing, is that expected?

That was supposed be added to the migration survey release, that's why changelog is not added, I'll add the changelog now.