location on http://checkstyle.sourceforge.net/ is not secure.

All checkstyle version below 8.18 are not very secure, see details at #6474.

I will move DTDs to another folder to let user experience failure and find this issue as request to update configuration.

Upgrade is simple:
http://checkstyle.sourceforge.net/dtds/configuration_1_3.dtd
=>
https://checkstyle.org/dtds/configuration_1_3.dtd
and
http://puppycrawl.com/dtds/configuration_1_3.dtd
=>
https://checkstyle.org/dtds/configuration_1_3.dtd

If you can not update, please write a comment, I might return DTDs to original place to give people some time to migrate.