close old findings: don't overwrite mitigated timestamp (#12204)

Author: valentijnscholten

dojo/importers/base_importer.py

@@ -752,7 +752,8 @@ def mitigate_finding(
         """
         finding.active = False
         finding.is_mitigated = True
-        finding.mitigated = self.scan_date
+        if not finding.mitigated:
+            finding.mitigated = self.scan_date
         finding.mitigated_by = self.user
         finding.notes.create(
             author=self.user,