Add preload hint for GitLab Sans italic

What does this MR do and why?

Add preload hint for GitLab Sans italic font

Gives browsers early notice that this font should be fetched and cached, making it more likely to be used and rendered with a cold cache.

This was missed in the merge request which added this font: !123936 (merged).

Related to #410199 (closed).

Changelog: fixed

Screenshots or screen recordings

n/a

How to set up and validate locally

1. Load any page.
2. View Source (usually via something like CtrlU).
3. Search for a <link> tag which has as="font" and rel="preload" which points to the actual GitLab Sans italic font file.

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 #410199 (closed)