Enable `new_fonts` feature flag by default

What does this MR do and why?

It enables the FF new_fonts by default.

How to set up and validate locally

In Chrome open the dev console and reload the page. Under the fonts tab under Network tab, you should see that new fonts - GitLabSans and 4 types of JetBrainsMono - were loaded

Inspect non-monospace text on the page - it should be rendered with GitLab Sans.

The monospace text should be rendered with JetBrainsMono.

Related to #379147 (closed)