Edit our upgrade documentation to include warnings related to scripts on Jira

Problem Definition

When upgrading Jira, customers may face issues related to scripts in the Announcement Banner or custom fields from Jira. Due to changes on Jira and its plugins, keeping scripts on Jira may cause certain pages to break, and we have no explicit warnings regarding that.

Suggested Solution

Update our documentation to alert about this behavior, avoiding possible issues during upgrades.