fix(PersistenceConfig): store UTC time in database

solidify-controller/src/main/java/ch/unige/solidify/config/PersistenceConfig.java

 package ch.unige.solidify.config;
 
 import java.time.OffsetDateTime;
+import java.time.ZoneOffset;
 import java.util.Optional;
 
 import org.springframework.context.annotation.Bean;
@@ -17,7 +18,7 @@ public class PersistenceConfig {
 
   @Bean // Makes OffsetDateTime compatible with auditing fields
   public DateTimeProvider auditingDateTimeProvider() {
-    return () -> Optional.of(OffsetDateTime.now());
+    return () -> Optional.of(OffsetDateTime.now(ZoneOffset.UTC));
   }
 
 }