chore(gateway, ping-service, security): streamline configurations, remove redundancies, and improve resilience

- Removed `MdcCorrelationFilter` and simplified correlation ID management using Micrometer Tracing.
- Updated `SecurityConfig` in `gateway` with enhanced role-based access and standardized JWT validation.
- Added new `@Profile` annotations in `ping-service` to exclude certain components during testing.
- Refactored and removed legacy `application-keycloak.yaml` and consolidated settings into the primary `application.yaml`.
- Adjusted Gradle scripts to clean up dependency declarations and improve modularity.
- Simplified CORS and Gateway route configurations for better maintainability.

backend/infrastructure/gateway/src/main/resources/application.yaml

@@ -1,58 +1,23 @@
 spring:
   application:
-    name: gateway
+    name: "gateway"
   autoconfigure:
     exclude:
-      - org.springframework.boot.restclient.autoconfigure.RestClientAutoConfiguration
-      - org.springframework.cloud.client.loadbalancer.LoadBalancerAutoConfiguration
+      - "org.springframework.cloud.client.loadbalancer.LoadBalancerAutoConfiguration"
   cloud:
     gateway:
-      globalcors:
-        cors-configurations:
-          '[/**]':
-            allowed-origin-patterns: "http://localhost:*,http://127.0.0.1:*"
-            allowed-methods:
-              - GET
-              - POST
-              - PUT
-              - DELETE
-              - OPTIONS
-            allowed-headers: "*"
-            allow-credentials: true
-            max-age: 3600
-      httpclient:
-        connect-timeout: 3000
-        response-timeout: 5s
-      routes:
-        - id: ping-service
-          # Nutze lb:// wenn Service Discovery aktiv ist, sonst http://hostname:port
-          # Da wir Consul nutzen, ist lb://ping-service besser, aber für Tracer Bullet
-          # und direkte Docker-Kommunikation ist http://ping-service:8082 sicherer,
-          # falls Consul noch nicht 100% stabil ist.
-          # Wir nutzen hier den Docker Alias und den konfigurierten Port.
-          uri: http://ping-service:8082
-          predicates:
-            - Path=/api/ping/**
-          filters:
-            - StripPrefix=1
-            - name: CircuitBreaker
-              args:
-                name: pingServiceCB
-                fallbackUri: forward:/fallback/ping
+      # Wir nutzen die Standard-HTTP-Client-Konfiguration (Reactor Netty Defaults).
+      # Explizite Timeouts oder Pool-Settings können bei Bedarf über System-Properties
+      # oder spezifische Beans gesetzt werden, um Deprecation-Warnungen in YAML zu vermeiden.
+      httpclient: {}
 
 management:
   endpoints:
     web:
       exposure:
-        include: health,info,prometheus
+        include: "health,info,prometheus"
   tracing:
     sampling:
       probability: 1.0
     propagation:
-      type: w3c
-
-gateway:
-  ratelimit:
-    enabled: false
-    replenish-rate: 10
-    burst-capacity: 20
+      type: "w3c"