Lint: disallow diff as code fence language in favour of syntax highlighting diffs

[IgnaceMaes]

In the following PR, code block fences with diff as language were refactored to use the data-diff attributes of the syntax highlighter: #1385

Linting should be added against diff as language to not allow any new usages.

Bad

\```diff
- some deletion
+ some addition
\```

Good

\```js {data-diff="-1,+2"}
some deletion
some addition
\```