Update StrongSwan to 5.9.11
https://github.com/strongswan/strongswan/releases/tag/5.9.11
Currently we are using 5.9.8, and there is some security fixed is 5.9.10 related to certificate validation.
But since 5.9.11 has been released, and fixes some regressions introduced in 5.9.10, I have selected that as the target version instead.