Update Figma Next unpublish date in Get Started (#4914)

Author: bhoppers2008

site/docs/getting-started/designing/index.mdx

@@ -17,7 +17,7 @@ New Salt DS Figma libraries are live! Plan your migration now to ensure a seamle
 - Existing libraries retired: Immediately after release
 - Existing libraries unpublished
   - Light/ Dark Figma Libraries (non-variable based, UITK themed): February 28th
-  - Salt (Next) Libraries (variable based, Brand themed): March 28th
+  - Salt (Next) Libraries (variable based, Brand themed): May 2nd
 
 For more details, read our [Migration Guide](https://go/salt-ds-internal/migration-strategies/salt-ds-figma-libraries/index). Have questions? Visit our [support page](https://go/salt-ds-internal/support) or [contact us](/salt/support-and-contributions/index).