Refactor migration into two separate migrations for v21.0.0

Co-authored-by: damyanpetev <[email protected]>

Author: Copilot

projects/igniteui-angular/migrations/migration-collection.json

@@ -249,9 +249,14 @@
         },
         "migration-50": {
             "version": "21.0.0",
-            "description": "Updates Ignite UI for Angular from v20.1.0 to v21.0.0 - migrates to multiple entry points",
-            "factory": "./update-21_0_0",
-            "schema": "./update-21_0_0/schema.json"
+            "description": "Updates Ignite UI for Angular from v20.1.0 to v21.0.0",
+            "factory": "./update-21_0_0"
+        },
+        "migration-51": {
+            "version": "21.0.0",
+            "description": "Migrates imports to use granular entry points for better tree-shaking",
+            "factory": "./update-21_0_0_import-migration",
+            "optional": true
         }
     }
 }