Enable highlight_js_worker feature flag by default

What does this MR do and why?

Enables the highlight_js_worker feature flag by default as it is currently globally enabled on gitlab.com.

Screenshots or screen recordings

No visual changes are expected.

How to set up and validate locally

Viewing source code via the repository blob viewer should work exactly as it did before.

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 #415755 (closed)