Enable new rich text to source toggle button in self-managed instances by default

What does this MR do and why?

This MR enables by default the feature flag that displays the new toggle button to switch between source and rich text modes in the Wikis on self-managed instances.

This feature is already enabled globally on gitlab.com

Screenshots or screen recordings

Self-managed instances will see this new UI by default

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #345398 (closed)