chore(marketplace): declare Node engines + drop obsolete COPYRIGHT_NOTICE

Preparing for submission to market.strapi.io. Two small alignments on
top of the existing (already compliant) metadata:

- package.json
  * engines: declared node >=18 <=22 and npm >=6 to match the Strapi 5
    compatibility matrix. Marketplace reviewers flag missing engines
    because installs on unsupported Node versions silently misbehave.
  * files: removed COPYRIGHT_NOTICE.txt so the published tarball no
    longer ships a file that does not exist in the source tree.

- COPYRIGHT_NOTICE.txt: removed. It was a leftover metadata file from
  when LICENSE carried the 'ADDITIONAL CONDITION' clause. With the
  LICENSE now being the canonical SPDX MIT text, the separate notice
  file only adds confusion — the source of truth is singular.

No runtime or public-API change. Both `strapi-plugin build` and
`strapi-plugin verify` pass.

Author: Schero94

COPYRIGHT_NOTICE.txt

@@ -35,9 +35,13 @@
     "strapi-server.js",
     "README.md",
     "LICENSE",
-    "COPYRIGHT_NOTICE.txt",
+    "CHANGELOG.md",
     "pics"
   ],
+  "engines": {
+    "node": ">=18.0.0 <=22.x.x",
+    "npm": ">=6.0.0"
+  },
   "scripts": {
     "build": "strapi-plugin build",
     "watch": "strapi-plugin watch",