Failed to upgrade to 24.04

[eresquigal]

CMD: do-release-upgrade -d
Fails.
----
# lsb_release -rd
No LSB modules are available.
Description: Ubuntu 23.10
Release: 23.10
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ii ubuntu-release-upgrader-core 1:23.10.14
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Expected it to upgrade to 24.04
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Instead: Pulls the package lists, then after a while says:

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

This was caused by:
* Upgrading to a pre-release version of Ubuntu

ProblemType: Bug
DistroRelease: Ubuntu 23.10
Package: ubuntu-release-upgrader-core 1:23.10.14
ProcVersionSignature: Ubuntu 6.5.0-35.35-generic 6.5.13
Uname: Linux 6.5.0-35-generic x86_64
NonfreeKernelModules: zfs nvidia_modeset nvidia
ApportVersion: 2.27.0-0ubuntu5
Architecture: amd64
CasperMD5CheckResult: unknown
CrashDB: ubuntu
Date: Wed May 15 17:38:52 2024
InstallationDate: Installed on 2016-02-28 (2999 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Release amd64 (20151021)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to mantic on 2024-05-15 (0 days ago)
VarLogDistupgradeAptHistorylog:
 Start-Date: 2024-05-15 17:36:36
 End-Date: 2024-05-15 17:36:36
VarLogDistupgradeApttermlog:
 Log started: 2024-05-15 17:36:36
 Log ended: 2024-05-15 17:36:36