Sanity Studio v4.21.1: Clarify React 19 CLI message, minor fixes

🐛 Notable bugfixes

Fixes suggested upgrade command for deprecated packages to use caret (^) version range instead of exact version

Marks React < 19.2.1 as deprecated

Fixes a bug where blueprints doctor would incorrectly throw an error when blueprints config files were not present

Fixes a regression in v4.20.0 that caused icons in dashboard to miss text styling

Fixes an issue where diffs author and timestamp were missing for operations which removed data, for example, changing a title from summer week to summer