update-grub fails to set submenu item as default by name

[TomyLobo]

# Motivation
The latest 22.04 HWE kernel (6.5.0-41) doesn't work with my dock (#2070020), so I'm trying to set a previous stable kernel (6.5.0-35) as a default in GRUB.

# What I did
I've added the following to /etc/default/grub.d/pin-6.5.0-35.cfg:
```
GRUB_DEFAULT="Ubuntu, with Linux 6.5.0-35-generic"
```

# What went wrong
update-grub warns:
```
Warning: Please don't use old title `Ubuntu, with Linux 6.5.0-35-generic' for GRUB_DEFAULT, use `Advanced options for Ubuntu>Ubuntu, with Linux 6.5.0-35-generic' (for versions before 2.00) or `gnulinux-advanced-abd4adac-c96a-4e9b-bc6b-17edaad709ee>gnulinux-6.5.0-35-generic-advanced-abd4adac-c96a-4e9b-bc6b-17edaad709ee' (for 2.00 or later)
```
It does add an override to grub.cfg to change the default to "Advanced options for Ubuntu>Ubuntu, with Linux 6.5.0-35-generic", though:
```
if [ "x$default" = 'Ubuntu, with Linux 6.5.0-35-generic' ]; then default='Advanced options for Ubuntu>Ubuntu, with Linux 6.5.0-35-generic'; fi;
```
That's why I initially thought I did everything right and disregarded the warning.

# Workaround
I changed /etc/default/grub.d/pin-6.5.0-35.cfg to the following:
```
GRUB_DEFAULT="Advanced options for Ubuntu>Ubuntu, with Linux 6.5.0-35-generic"
```

# Remarks
The workaround is the preferred way to set a default by name.
However, I believe that if there is code that generates a broken override, it should be fixed or removed.

# Relation to other bugs
I believe this is NOT a duplicate of #2049681, since it also occurs without "GRUB_DISABLE_SUBMENU=true", but depending on what the issue was with #2049681, that bug might be covered by a solution for mine, so a "duplicates" pointing in the other direction might be more appropriate, if I understand the meaning of "duplicates" correctly.

# Causes
grub-mkconfig generates broken grub.cfg code to add the submenu part to the default.
It's still present in the current master branch:
https://git.savannah.gnu.org/gitweb/?p=grub.git;a=blob;f=util/grub.d/10_linux.in;h=cc393be7eda9ab79cd0207434924c06fbd27bb6d;hb=HEAD#l104
It's missing an `x` before `$quoted`.
Because of that, the condition always fails and GRUB falls back to booting the first available kernel.

This was introduced when grub-mkconfig was updated to support submenus in 2012:
https://git.savannah.gnu.org/gitweb/?p=grub.git;a=commitdiff;h=0cdc126ca2c7af914ede99253d1546ebbd11e937

ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: grub2 (not installed)
ProcVersionSignature: Ubuntu 6.5.0-35.35~22.04.1-generic 6.5.13
Uname: Linux 6.5.0-35-generic x86_64
NonfreeKernelModules: zfs
ApportVersion: 2.20.11-0ubuntu82.5
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: KDE
Date: Mon Jun 24 11:41:59 2024
InstallationDate: Installed on 2023-07-05 (354 days ago)
InstallationMedia: Kubuntu 22.04.2 LTS "Jammy Jellyfish" - Release amd64 (20230223)
SourcePackage: grub2
UpgradeStatus: No upgrade log present (probably fresh install)