do-release-upgrade refuses to upgrade due to "An unresolvable problem occurred while calculating the upgrade"

[Christopher Barrington-Leigh]

do-release-upgrade on a 23.04 system says:

```
Calculating the changes

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

If none of this applies, then please report this bug using the
command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal. If
you want to investigate this yourself the log files in
'/var/log/dist-upgrade' will contain details about the upgrade.
Specifically, look at 'main.log' and 'apt.log'.
```

main.log has no details I can follow, but near the end says:

```
Dist-upgrade failed: 'E:Unable to correct problems, you have held broken packages.'
```

I have no held packages of my own.
There are no PPAs listed in sources.list.d
There is nothing except for lunar repositories in sources.list
There seems to be no mention of a specific package I can address.

I am stumped.

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: pass
CrashDB: ubuntu
CrashReports:
 640:1001:124:27461133:2024-01-02 12:35:58.772439611 -0500:2024-01-02 12:35:58.788439801 -0500:/var/crash/_usr_bin_evince.1001.crash
 600:117:124:37:2024-01-02 12:36:19.196682163 -0500:2024-01-02 12:36:19.196682163 -0500:/var/crash/_usr_bin_evince.1001.uploaded
 644:1001:124:0:2024-01-02 12:35:58.772439611 -0500:2024-01-02 12:35:58.772439611 -0500:/var/crash/_usr_bin_evince.1001.upload
CurrentDesktop: ubuntu:GNOME
Date: Tue Jan 2 19:34:47 2024
EcryptfsInUse: Yes
InstallationDate: Installed on 2022-09-02 (487 days ago)
InstallationMedia: Ubuntu 22.04.1 LTS "Jammy Jellyfish" - Release amd64 (20220809.1)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to lunar on 2024-01-03 (0 days ago)