Code blocks with "diff" language get no syntax highlighting

[hvt]

Looking at the new docs, I found that code blocks with diff as a language, were not syntax highlighted. See https://symfony.com/doc/current/security.html#form-login for example (the third code block showing the LoginController diff).

[javiereguiluz]

You are right. This was reported before (see #15963 (comment)) and we're working on a fix. Thank you!