Some more (background) info about this / the reasons for this message from PostgreSQL / operating system (Debian) side is available here:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1029548#10
The cause for that (IIUC) is that the database was created when libc6 was at version 2.35 and it has since been upgraded to 2.36.
See also https://bugs.debian.org/1021074
snip
A (major) libc6 upgrade is not something that will happen on Stable, so this issue may only occur with people running Testing or Unstable.
and
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1029548#20
But it will happen when people upgrade their stable machines to bookworm once it is released.
AFAIK, that upgrade procedure is different and those users will not run into this issue.
Disclaimer for the future (e.g. arriving via a search engine here):
While it seems to be currently “safe” do just raise / update the collation on further glibc upgrades additional checks / research and evaluation of the database might be required: