do not migrate preflight in non-v3 projects

Author: RobinMalfait

packages/@tailwindcss-upgrade/src/codemods/css/migrate-preflight.ts

@@ -5,6 +5,7 @@ import type { Config } from '../../../../tailwindcss/src/compat/plugin-api'
 import type { DesignSystem } from '../../../../tailwindcss/src/design-system'
 import { toKeyPath } from '../../../../tailwindcss/src/utils/to-key-path'
 import * as ValueParser from '../../../../tailwindcss/src/value-parser'
+import * as version from '../../utils/version'
 
 // Defaults in v4
 const DEFAULT_BORDER_COLOR = 'currentcolor'
@@ -46,6 +47,10 @@ export function migratePreflight({
   }
 
   function migrate(root: Root) {
+    // CSS for backwards compatibility with v3 should only injected in v3
+    // projects and not v4 projects.
+    if (!version.isMajor(3)) return
+
     let isTailwindRoot = false
     root.walkAtRules('import', (node) => {
       if (