update-grub fails to set default menu item when submenu is disabled

[Pavel Malyshev]

Since latest 22.04 HWE kernel crashes, I'm trying to set a previous stable kernel as a default in GRUB.
I've added the following to /etc/default/grub:
```
GRUB_DEFAULT="Ubuntu, with Linux 6.2.0-39-generic"
GRUB_DISABLE_SUBMENU=true
```

I have to disable submenu, since my GRUB is password protected. So when I just set GRUB_DEFAULT, GRUB blocks asking for user and password.

When I both set default kernel and disable submenu, update-grub complains:
```
Warning: Please don't use old title `Ubuntu, with Linux 6.2.0-39-generic' for GRUB_DEFAULT, use `Advanced options for Ubuntu>Ubuntu, with Linux 6.2.0-39-generic'
```
update-grub also adds an override to the default menu item in grub.cfg to look like "Advanced options for Ubuntu>Ubuntu, with Linux 6.2.0-39-generic", so GRUB boots first available kernel instead of the default one.

ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: grub-common 2.06-2ubuntu7.2
ProcVersionSignature: Ubuntu 6.2.0-39.40~22.04.1-generic 6.2.16
Uname: Linux 6.2.0-39-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu82.5
Architecture: amd64
CasperMD5CheckResult: pass
CloudArchitecture: x86_64
CloudID: none
CloudName: none
CloudPlatform: none
CloudSubPlatform: config
Date: Wed Jan 17 14:19:05 2024
InstallationDate: Installed on 2023-07-02 (198 days ago)
InstallationMedia: Ubuntu-Server 22.04.2 LTS "Jammy Jellyfish" - Release amd64 (20230217.1)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=C.UTF-8
 SHELL=/bin/bash
SourcePackage: grub2
UpgradeStatus: No upgrade log present (probably fresh install)
modified.conffile..etc.grub.d.10_linux: [modified]
modified.conffile..etc.grub.d.40_custom: [modified]
mtime.conffile..etc.grub.d.10_linux: 2024-01-17T14:13:44.311467
mtime.conffile..etc.grub.d.40_custom: 2023-07-03T15:45:29.826960