Add a versionadded notice

Author: mpdude

configuration/environment_variables.rst

@@ -486,6 +486,10 @@ Symfony provides the following env var processors:
                 'auth' => '%env(require:AUTH_FILE)%',
             ]);
 
+    .. versionadded:: 4.3
+
+        The ``require`` processor was introduced in Symfony 4.3.
+
 ``env(trim:FOO)``
     Trims the content of ``FOO`` env var, removing whitespaces from the beginning
     and end of the string. This is especially useful in combination with the