Added diff_url in Wiki Page Hook

What does this MR do and why?

Describe in detail what your merge request does and why.

I have verified in my Webhook receiver that Wiki Page Webhook event sends diff_url field as below:

"diff_url":"https://gitlab.com/smit-modi/test/-/wikis/My-first-wiki-page/diff?version_id=78ee4a6705abfbff4f4132c6646dbaae9c8fb6ec"}}

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