AI Engine Pro 2.6.8

* Fix: Avoid deprecation issues with PHP 8.2+.

* Fix: The value of external select fields was not interpreted correctly.
* Fix: Better validation in the AI Forms.
* Fix: Empty chatbot shorcode will automatically switch to the default chatbot.
* Fix: Issues on mobile with the chatbot.
* Fix: Avoid displaying the rendered HTML in the chatbot when typed by the user.
* Update: Accept function calls without parameters, and make sure the types are correct.

* Add: New gpt-3.5-turbo-instruct model. Have a look at [this](https://openai.com/research/instruction-following).
* Fix: AI Forms work better with external input fields (such as radios and checkboxes). The debugging mode is also now more verbose.
* Update: Consolidate the way ID and Custom ID are handled. That fixes a few issues with custom chatbots too.