[ Bug]: Jira On-Prem provider resets the priority on every update

Describe the bug

JiraonpremProvider._notify (keep/providers/jiraonprem_provider/jiraonprem_provider.py:524) declares priority: str = "Medium", and the update branch hands that value to __update_issue (line 551). __update_issue writes the field whenever the value is truthy (lines 386-387):

if priority:
    update["priority"] = [{"set": {"name": priority}}]

So a step that updates a summary also sets the priority to Medium. A ticket somebody escalated by hand goes back to Medium the next time any workflow touches it, and nothing in that step mentioned priority.

The Jira Cloud provider does not do this. Its _notify has no priority parameter and passes none to its own __update_issue (keep/providers/jira_provider/jira_provider.py:546).

To Reproduce

1. Create a ticket through the on-prem provider.
2. Raise its priority in Jira to Highest.
3. Run a workflow action with issue_id and a new summary, saying nothing about priority.
4. The ticket is Medium again.

Expected behavior

An update changes the priority only when the workflow names one. Creating an issue keeps the Medium fallback, since the create screen carries the field.

Additional context

A PR implementing this follows.