Fix: Correct AJAX action for API tests and bump version to 2.1.2

- Fix: Dynamically set AJAX action in admin-script.js for API test buttons to ensure correct backend handler is called for DeepL, Google, and Microsoft.
- Fix: Set correct AJAX action for DeepL API key auto-check.
- Chore: Remove redundant admin-script-new.js.
- Chore: Update plugin version to 2.1.2 in cn-translate-slugs.php and readme.txt.
- Docs: Add changelog entry for version 2.1.2.

バージョン2.0.5リリース