Add `diff` language specification to `Podfile.lock` diff annotation

[mokagio]

In #59, @AliSoftware suggested adding diff to the code block in the Podfile.lock diverged failure annotation, to see if Buildkite also adds syntax highlighting to the rendered Markdown.

I implemented it here and made a test PR on Woo iOS. It doesn't look like there's any syntax highlighting in the annotation 😞

On the other hand, the diff in the code block is not rendered in the code, which makes me hopeful. Or, at least, it makes it look like an harmless change to add.

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• [x] I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.

[mokagio]

@AliSoftware super fast review. Thanks! So fast, in fact, that I think it arrived before the edits I made in the description in regards to the lack of syntax highlighting.

I think you'll agree with my conclusion that it's okay to merge anyway, but leaving a note about it, just in case.

[AliSoftware]

Unfortunate that there's no syntax highlighting after all, but doesn't hurt to merge anyway 👍