Revert github copilot to experimental, extend its description

As per discussion with the license management people as well as the AI product manager in the Qt Creator 15 M4 review (https://intranet.qt.io/pages/viewpage.action?spaceKey=QTPO&title=PM+Board+Meeting+minutes) and afterwards, the following changes are required: 

• replace disabledByDefault : true with experimental : true
• add a prefix sentence to the description field “As with most solutions that use Generative AI, the Github Copilot extension is considered experimental.”

Rationale: labeling copilot as experimental puts it under Pre-release Code as defined in Qt Development Framework. 

The legal need to require the user to accept terms & conditions for Qt Copilot shall be handled separately (and needs validation whether T&C are applicable in this instamce).