Upgrade to 23.10 failed with "unable to compute upgrade."

[Simon Bate]

This seems similar to #2043272, but not precisely.

My log shows:
2023-12-17 15:18:26,189 DEBUG Installing 'linux-generic' (linux metapackage may have been accidentally uninstalled)
2023-12-17 15:18:26,857 DEBUG Marking 'ubuntu-desktop' for upgrade
2023-12-17 15:18:27,265 WARNING Can't mark 'ubuntu-desktop' for upgrade (E:Unable to correct problems, you have held broken packages.)

ProblemType: Bug
DistroRelease: Ubuntu 23.04
Package: ubuntu-release-upgrader-core 1:23.04.7
ProcVersionSignature: Ubuntu 6.2.0-39.40-generic 6.2.16
Uname: Linux 6.2.0-39-generic x86_64
ApportVersion: 2.26.1-0ubuntu2.1
Architecture: amd64
CasperMD5CheckResult: unknown
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Sun Dec 17 15:19:30 2023
InstallationDate: Installed on 2023-09-04 (104 days ago)
InstallationMedia: Ubuntu 23.04 "Lunar Lobster" - Release amd64 (20230418)
PackageArchitecture: all
ProcEnviron:
 LANG=en_US.UTF-8
 PATH=(custom, no user)
 SHELL=/bin/bash
 TERM=xterm-256color
 XDG_RUNTIME_DIR=<set>
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to lunar on 2023-12-17 (0 days ago)
VarLogDistupgradeTermlog: