issues: document switch to usrmerge

This is an attempt to close https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=992116 by documenting the switch to 'usrmerge' in bookworm

i didnt reply to that bug, but my understanding is that things have moved on since it was reported in 2021: no longer any significant breakage to document and the upgrade path is now solved - however, i think worth including in the release notes as it is a big change for systems installed before buster