minor #14275 simple_preauth was removed in symfony 5.0 (dbu)

This PR was submitted for the 5.0 branch but it was merged into the 5.1 branch instead.

Discussion
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simple_preauth was removed in symfony 5.0

https://github.com/symfony/symfony/blob/master/UPGRADE-5.0.md#securitybundle

while trying to find out how to migrate, i stumbled over this page which still mentions simple_preauth even though the option was removed

Commits
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295c688 simple_preauth was removed in symfony 5.0

Author: javiereguiluz

reference/configuration/security.rst

@@ -395,8 +395,6 @@ depend on the authentication mechanism, which can be any of these:
                         # ...
                     remote_user:
                         # ...
-                    simple_preauth:
-                        # ...
                     guard:
                         # ...
                     form_login:
@@ -405,8 +403,6 @@ depend on the authentication mechanism, which can be any of these:
                         # ...
                     json_login:
                         # ...
-                    simple_form:
-                        # ...
                     http_basic:
                         # ...
                     http_basic_ldap: