Migrate to shared alert in label_page_create

What does this MR do and why?

Migrate to shared alert in app/views/shared/issuable/_label_page_create.html.haml.

Part of &5213 (closed).

Screenshots or screen recordings

before	after

How to set up and validate locally

1. Navigate to a new issue page
2. In "Labels", click the dropdown then click "Create project label"
3. Add any name and long random string for the color field
4. Click "Create"
5. Observe the error alert

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #354592 (closed)