add an issue about sysctl.conf getting disabled

This is a regression and a surprise that has left multiple professional sysadmins in my filter bubble without routing on upgrade. The settings in /etc/sysctl.conf do not seem to get preserved, linux-sysctl-defaults installs a new defaults file, which will, for example have ip forwarding turned off.

This is being discussed in #1110184 with the major opinion being "well, you're holding it wrong", but I think this should be documented in the release notes, and it should be in the release notes for the point release 13.1.