[Doctrine] Warn about DBAL 4.x changes for MariaDB connection

Author: javiereguiluz

reference/configuration/doctrine.rst

@@ -109,7 +109,9 @@ The following block shows all possible configuration keys:
     version).
 
     If you are running a MariaDB database, you must prefix the ``server_version``
-    value with ``mariadb-`` (e.g. ``server_version: mariadb-10.4.14``).
+    value with ``mariadb-`` (e.g. ``server_version: mariadb-10.4.14``). This will
+    change in Doctrine DBAL 4.x, where you must define the version as output by
+    the server (e.g. ``10.4.14-MariaDB``).
 
     Always wrap the server version number with quotes to parse it as a string
     instead of a float number. Otherwise, the floating-point representation