Release: v12.1.0. [skip ci]

package.json

 {
   "name": "@ckeditor/ckeditor5-build-decoupled-document",
-  "version": "12.0.0",
+  "version": "12.1.0",
   "description": "The document editor build of CKEditor 5 – the best browser-based rich text editor.",
   "keywords": [
     "ckeditor5-build",