Allow for non-admin CSV import to update existing issues based on issue ID

Problem Definition

Currently there are two ways of importing issues:

1. Import issues from CSV (accessible by admins and non-admins)
2. External system import

Option 1 does not allow to update existing issues, and instead will always create new issues. This is because the Issue Key field is not mappable from this tool (see JRACLOUD-62865).

In contrast, Issue ID is mappable from this tool. If the Issue ID field was usable by Jira to map to existing issues, and allow for them to be updated in-place, then non-admins would have a tool to update existing issues via CSV import, where they currently have none.

Suggested Solution

Allow use of the Issue ID field during CSV import to map to existing issues, and update existing issues in-place.