Update auth certs readme

tests/testdata/auth_system/README.md

@@ -1,5 +1,13 @@
 # QGIS Test Certificates/Keys
 
+Since 2021 the test certificates are generated by a script in `certs_keys_2048`
+directory because the old certificates were using 1024 keys that are not
+accepted by openssl anymore.
+
+## Old certificates
+
+The following paragraphs describe the old certificates implementation.
+
 The certs/keys are generated/edited using **XCA** (see xca-project directory):
 
     https://sourceforge.net/projects/xca/