docs: add forceEsbuild removal to migration guide - Document the breaking change for custom-esbuild users - Provide before/after examples for angular.json configuration - Explain that forceEsbuild is no longer needed since @angular/build uses esbuild by default

Author: just-jeb

MIGRATION.MD

@@ -2,7 +2,28 @@
 
 ## Custom ESBuild builder
 
-- No breaking changes (except for updating to Angular 20)
+1. The `forceEsbuild` property has been removed from the dev-server configuration. This property is no longer needed since the builder now uses `@angular/build` directly, which uses esbuild by default.
+
+   **Before:**
+   ```json
+   "serve": {
+     "builder": "@angular-builders/custom-esbuild:dev-server",
+     "options": {
+       "forceEsbuild": true,
+       "port": 5006
+     }
+   }
+   ```
+
+   **After:**
+   ```json
+   "serve": {
+     "builder": "@angular-builders/custom-esbuild:dev-server",
+     "options": {
+       "port": 5006
+     }
+   }
+   ```
 
 ## Custom Webpack builder