Update /#hooks documentation to reflect the current schema

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Description

As we've recently updated our project and group webhooks, we need to update API documentation and our OpenAPI spec to reflect the current schema.

Documentation:

• https://docs.gitlab.com/ee/api/projects.html#hooks
• https://docs.gitlab.com/ee/api/groups.html#hooks

Schemata:

• Group webhooks: https://gitlab.com/gitlab-org/gitlab/blob/master/ee/spec/fixtures/api/schemas/public_api/v4/group_hook.json
• Project webhooks: https://gitlab.com/gitlab-org/gitlab/blob/master/spec/fixtures/api/schemas/public_api/v4/project_hook.json