upgrade to PostgreSQL 16

Author: deki

labs/unicorn-store/infrastructure/cdk/src/main/java/com/unicorn/core/InfrastructureStack.java

@@ -68,7 +68,7 @@ private SecurityGroup createDatabaseSecurityGroup(IVpc vpc) {
     private DatabaseInstance createRDSPostgresInstance(IVpc vpc, DatabaseSecret databaseSecret) {
 
         var databaseSecurityGroup = createDatabaseSecurityGroup(vpc);
-        var engine = DatabaseInstanceEngine.postgres(PostgresInstanceEngineProps.builder().version(PostgresEngineVersion.VER_15).build());
+        var engine = DatabaseInstanceEngine.postgres(PostgresInstanceEngineProps.builder().version(PostgresEngineVersion.VER_16).build());
 
         return DatabaseInstance.Builder.create(this, "UnicornInstance")
                 .engine(engine)