Fix work item issues webhook event type and object kind

What does this MR do and why?

Fixes Ensure event type and object kind is issue for ... (#517947 - closed)

Fix work item issues webhook event type and object kind

Ensure that issue event webhooks for work items of type issue use issue for both event_type and object_kind to prevent us from introducing a breaking change for customer integrations.

Changelog: fixed

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• Ensure event type and object kind is issue for ... (#517947 - closed)
• https://gitlab.com/gitlab-org/gitlab/-/issues/512715#note_2336324625

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Set up webhook for "issue events" and e.g. use an online service such as https://webhook.site/ as the URL.
2. Create an issue and use the new issue experience and change parameters. You should see webhook events with object_kind and event_type issue`
3. Do the same for work items of type Task. ou should see webhook events with object_kind and event_type work_item`
4. Optionally, check the same with the legacy issue experience. You should see webhook events with object_kind and event_type issue`