Remove our custom diff parsing and prefer Github compare json API

[Urgau]

Follow-up to #2005, only the last commit is relevant.

[rustbot]

⚠️ Warning ⚠️

• Some commits in this PR modify submodules.

[Urgau]

• Some commits in this PR modify submodules.

This is really weird, we don't have any submodules in this repository. For some reason my commit probably has two parents, maybe the rebase was weird, idk

btw, this is read for review @Kobzol

[Urgau]

Oh, this is funny, our modified submodules handler is looking at the PR diff for "Subproject commit" in it and warns if it finds one, which of course there are, bc I'm modifying the handler tests. 😄

triagebot/src/handlers/check_commits/modified_submodule.rs

Lines 7 to 8 in c335263

	let re = regex::Regex::new(r"\+Subproject\scommit\s").unwrap();
	if diff.iter().any(|fd| re.is_match(&fd.diff)) {