fix: frontend/package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-ANGULARCORE-15610287

frontend/package.json

@@ -20,7 +20,7 @@
     "@angular/cdk": "21.1.3",
     "@angular/cdk-experimental": "21.1.3",
     "@angular/common": "21.1.3",
-    "@angular/core": "21.1.3",
+    "@angular/core": "21.2.3",
     "@angular/forms": "21.1.3",
     "@angular/localize": "21.1.3",
     "@angular/platform-browser": "21.1.3",