When updating mirror repo intervals by API reschedule next update too (#19429) (#19433)

Backport #19429

When a mirror repo interval is updated by the UI it is rescheduled with that interval
however the API does not do this. The API also lacks the enable_prune option.

This PR adds this functionality in to the API Edit Repo endpoint.

Signed-off-by: Andrew Thornton <art27@cantab.net>

templates/swagger/v1_json.tmpl

@@ -14757,6 +14757,11 @@
           "type": "string",
           "x-go-name": "Description"
         },
+        "enable_prune": {
+          "description": "enable prune - remove obsolete remote-tracking references",
+          "type": "boolean",
+          "x-go-name": "EnablePrune"
+        },
         "external_tracker": {
           "$ref": "#/definitions/ExternalTracker"
         },