testing/apache-mod-auth-openidc: upgrade to 2.4.15.6
https://github.com/OpenIDC/mod_auth_openidc/releases/tag/v2.4.15.6
https://github.com/OpenIDC/mod_auth_openidc/compare/v2.4.15.5...v2.4.15.6
Release tarball for the previous version has been deleted: https://build.alpinelinux.org/buildlogs/build-edge-x86/testing/apache-mod-auth-openidc/apache-mod-auth-openidc-2.4.15.5-r0.log