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

[purisev]

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.