Hide migration nudges for legacy WCS&T merchants

[kallehauge]

Description

Although new users of the feature has been disabled for about 6 years, then we still have some WCS&T merchants that user our live rate Shipping Method. Migrating them to WC Shipping will cause them to lose their shipping methods and thereby, potentially, not charge any shipping fees in checkout (read: they might not have other alternative shipping methods in place).

This PR aims to hide the migration banner so we can reach out to them via email and share custom path forward to reach feature parity.

Related issue(s)

• pevPIe-PB-p2

Steps to reproduce

1. Checkout trunk locally
2. Delete the wcshipping_migration_state option
3. Make sure WC Shipping isn't activated
4. Follow the instructions in the WCS PR no. 2590 to enable legacy shipping methods on your site
5. Go to "WooCommerce > Settings > Shipping > Shipping Zones" (/wp-admin/admin.php?page=wc-settings&tab=shipping)
6. Edit (or add) a shipping zone
7. Add a WCS&T shipping method to any zone (the settings doesn't matter)
8. Refresh the page
9. Verify you see the migration banner from the screenshot section
10. Checkout this branch locally
11. Refresh the page
12. Verify the banner is no longer showing
13. Go to "WooCommerce > Settings > Shipping > Shipping Zones" (/wp-admin/admin.php?page=wc-settings&tab=shipping)
14. Delete all WCS&T shipping methods
15. Refresh the page
16. Verify you're seeing the banner again

Bonus: if you repeat the steps above but with the feature announcement instead (read: the announcement we show when opening the "Purchase shipping labels" modal).

Screenshots/GIFs

Checklist

• unit tests

• changelog.txt entry added
• readme.txt entry added

[Ferdev]

Works as expected, and code looks good! 🚀