How to avoid critical updates prompt when admin login everytime?
I don't think there is there is option or configuration to to disable critical updates. It not lack of functionality but its by design.
We should understand following points:
Critical updates are critical
Salesforce analyses your organisation to determine if a critical update potentially affects your customization. If your customization are not affected, Salesforce automatically activates the update in your organisation. So if it hasn’t auto-activated you need to resolve issues!
On the scheduled auto-activation date, Salesforce permanently activates the update. After auto-activation, you cannot deactivate the update.
So, we should follow best practice of applying Critical updates asap and make sure your code works properly with new updates.