Updating postgres test container to v16

src/integrationTest/java/uk/gov/hmcts/darts/testutils/PostgresIntegrationBase.java

@@ -43,7 +43,7 @@ public class PostgresIntegrationBase {
     private static final int SERVER_MAX_CONNECTIONS = 50;
 
     private static final PostgreSQLContainer<?> POSTGRES = new PostgreSQLContainer<>(
-        "postgres:15-alpine"
+        "postgres:16-alpine"
     ).withDatabaseName("darts")
         .withUsername("darts")
         .withPassword("darts");