Add warnings to UPM upgrade pop-up and UPM update button

Problem Definition

As a Jira Server admin I do not want to be encouraged to perform operations which can have destructive results on my Jira instance like:

• clicking yes to UPM upgrade pop-up.
• clicking update UPM button
  in order to prevent the apps being unintentionally restarted and unavailable for my end users.

Suggested Solution

Possible solutions:

• remove the upgrade UPM pop-up
• add warnings in the UI after clicking the upgrade button
• add confirmation pop-up with warnings.