Update Codescanning Webhook Docs to Include Default Sender Field

[natashaU]

Why:

We're adding GitHub as the default sender field in the code-scanning alert payload.

What's being changed:

Hi there 👋

I'm from the codescanning Team at GitHub and we're doing a slight update. Previously:

sender	object	If the action is reopened_by_user or closed_by_user, the sender object will be the user that triggered the event. The sender object is empty for all other actions.

See here for more info: https://docs.github.com/en/developers/webhooks-and-events/webhook-events-and-payloads#code_scanning_alert

Now, we'll include a default sender object instead of having that field be empty and want the docs to reflect the default field value.

Related Links:

payload example PR: octokit/webhooks#366 (comment)

cc/ @bogdanap @anaarmas @krukow

Check off the following:

• I have reviewed my changes in staging. (look for the deploy-to-heroku link in your pull request, then click View deployment)
• For content changes, I have reviewed the localization checklist
• For content changes, I have reviewed the Content style guide for GitHub Docs.

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