Replace `createFlash` for `createAlert`

What does this MR do and why?

Swapped deprecated createFlash function in app/assets/javascripts/activities.js for Pajamas compliant createAlert.

There is a small change in the look and feel.

Screenshots or screen recordings

Before

After

How to set up and validate locally

It is not simple to display the alert in http://127.0.0.1:3000/dashboard/activity. I copy both functions to the Activities constructor (app/assets/javascripts/activities.js) to see the differences.

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.