Add object_attributes.action for note webhooks

What does this MR do and why?

After !127169 (merged) note webhooks now also fire when a note is updated, as well as created.

As suggested in !127169 (comment 1822611315), this MR adds a new object_attributes.action property to the comment webhook to provide customers with a way to determine which event triggered the webhook. This is a property key used in other webhooks for the same purpose.

MR acceptance checklist

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How to set up and validate locally

1. Create a project webhook:
   1. Choose a project, go Settings > Webhooks.
   2. For URL use https://webhook.site to generate a new webhook receiving URL.
   3. For Trigger, select Comment events.
   4. Select Add webhook.
2. Choose an issue in the project.
   1. Add a comment to the issue. Your https://webhook.site URL should receive a webhook with object_attributes.action of "create".
   2. Now edit the same comment. Your https://webhook.site URL should receive a webhook with object_attributes.action of "update".