Add warning for obsolete keys

lib/Config.php

@@ -70,10 +70,25 @@ public function __construct(LoggerInterface $logger, IConfig $nextCloudConfigura
                 . self::CONFIG_KEY . ' which should contain the configuration '
                 . 'for the app user_backend_sql_raw.');
         }
+
+        $this->warnAboutObsoleteConfigKeys();
+    }
+
+    public function warnAboutObsoleteConfigKeys()
+    {
+        $obsolete_keys = array("db_type", "db_host", "db_port", "db_name", "mariadb_charset");
+        foreach ($obsolete_keys as $key) {
+            // not using getConfigValueOrFalse() here, because we want to also catch empty strings
+            if (array_key_exists(key: $key, array:$this->appConfiguration)) {
+                $this->logger->warning("The configuration key '{$key}' has been obsolete since "
+                    . "version 2.0.0, has no effect and can be removed.");
+            }
+        }
     }
 
     /**
-     * @return string db type to connect to
+     * @return string dsn to use for db connection
+     * @throws \UnexpectedValueException
      */
     public function getDsn()
     {