Set requires_webhook=false for wecom

What does this MR do and why?

What

Fix pipeline: https://jihulab.com/gitlab-cn/gitlab/-/pipelines/1354611/failures

Failure/Error: connection.public_send(...)
      ActiveRecord::RecordInvalid:
        Validation failed: Webhook can't be blank, Webhook must be a valid URL, Labels to be notified behavior is not included in the list, Please input valid Wecom group robot webhook key

Why

在 !1809 (merged) 引入了 wecom integration， integration 默认 webhook 是必需字段，但测试时没有传递此字段。

事实上 wecom integration 并不需要 webhook，所以我们应该将 webhook 设置为非必需字段。

def requires_webhook?
  false
end

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