Modify rule S6945: Upgrade severity and impact to Critical and HIGH

Following the documentation, MAINTAINABILITY issues should have at least HIGH
impact, and Critical severity, if they can lead to the introduction of bugs.

RELIABILITY issues should have at least HIGH impact, and Critical severity,
if they can for example cause corrupted data.

Both applies here. Hence, we upgrade the `defaultSeverity` and impact
levels.

These are the documentation link:
https://docs.sonarsource.com/sonarqube-server/2025.1/extension-guide/adding-coding-rules/#setting-severities-in-mqr-mode
https://docs.sonarsource.com/sonarqube-server/2025.1/extension-guide/adding-coding-rules/#setting-severities-in-se-mode

rules/S6945/jcl/metadata.json

@@ -8,7 +8,7 @@
   },
   "tags": [
   ],
-  "defaultSeverity": "Major",
+  "defaultSeverity": "Critical",
   "ruleSpecification": "RSPEC-6945",
   "sqKey": "S6945",
   "scope": "All",
@@ -16,8 +16,8 @@
   "quickfix": "unknown",
   "code": {
     "impacts": {
-      "RELIABILITY": "MEDIUM",
+      "RELIABILITY": "HIGH",
-      "MAINTAINABILITY": "MEDIUM"
+      "MAINTAINABILITY": "HIGH"
     },
     "attribute": "LOGICAL"
   }