Why is this kernel unsigned? [closed]

I am trying to downgrade the linux kernel from 6.5 to 6.2.I download the deb files from here and use this command:

sudo dpkg -i *.deb

Then I get the log as the following:

(Reading database ... 305419 files and directories currently installed.)Preparing to unpack linux-headers-6.2.0-060200_6.2.0-060200.202302191831_all.deb ...Unpacking linux-headers-6.2.0-060200 (6.2.0-060200.202302191831) over (6.2.0-060200.202302191831) ...Preparing to unpack linux-headers-6.2.0-060200-generic_6.2.0-060200.202302191831_amd64.deb ...Unpacking linux-headers-6.2.0-060200-generic (6.2.0-060200.202302191831) over (6.2.0-060200.202302191831) ...Preparing to unpack linux-image-unsigned-6.2.0-060200-generic_6.2.0-060200.202302191831_amd64.deb ...Unpacking linux-image-unsigned-6.2.0-060200-generic (6.2.0-060200.202302191831) over (6.2.0-060200.202302191831) ...Preparing to unpack linux-modules-6.2.0-060200-generic_6.2.0-060200.202302191831_amd64.deb ...Unpacking linux-modules-6.2.0-060200-generic (6.2.0-060200.202302191831) over (6.2.0-060200.202302191831) ...Setting up linux-headers-6.2.0-060200 (6.2.0-060200.202302191831) ...Setting up linux-headers-6.2.0-060200-generic (6.2.0-060200.202302191831) .../etc/kernel/header_postinst.d/dkms:

• dkms: running auto installation service for kernel 6.2.0-060200-generic...done.Setting up linux-modules-6.2.0-060200-generic (6.2.0-060200.202302191831) ...Setting up linux-image-unsigned-6.2.0-060200-generic (6.2.0-060200.202302191831) ...Processing triggers for linux-image-unsigned-6.2.0-060200-generic (6.2.0-060200.202302191831) .../etc/kernel/postinst.d/dkms:
• dkms: running auto installation service for kernel 6.2.0-060200-generic...done./etc/kernel/postinst.d/initramfs-tools:update-initramfs: Generating /boot/initrd.img-6.2.0-060200-genericI: The initramfs will attempt to resume from /dev/nvme0n1p8I: (UUID=00b95c2b-b6a7-46a3-851d-82802e787926)I: Set the RESUME variable to override this./etc/kernel/postinst.d/zz-update-grub:Sourcing file /etc/default/grub' Sourcing file /etc/default/grub.d/init-select.cfg'Generating grub configuration file ...Found linux image: /boot/vmlinuz-6.5.0-44-genericFound initrd image: /boot/initrd.img-6.5.0-44-genericFound linux image: /boot/vmlinuz-6.5.0-14-genericFound initrd image: /boot/initrd.img-6.5.0-14-genericFound linux image: /boot/vmlinuz-6.2.0-060200-genericFound initrd image: /boot/initrd.img-6.2.0-060200-genericWarning: Please don't use old title Ubuntu, with Linux 6.2.0-060200-generic' for GRUB_DEFAULT, use Advanced options for Ubuntu>Ubuntu, with Linux 6.2.0-060200-generic' (for versions before 2.00) or `gnulinux-advanced-1ea83245-64f1-4583-8f8f-64f0af3c6413>gnulinux-6.2.0-060200-generic-advanced-1ea83245-64f1-4583-8f8f-64f0af3c6413' (for 2.00 or later)Memtest86+ needs a 16-bit boot, that is not available on EFI, exitingWarning: os-prober will be executed to detect other bootable partitions.Its output will be used to detect bootable binaries on them and create new boot entries.Found Windows Boot Manager on /dev/nvme0n1p1@/EFI/Microsoft/Boot/bootmgfw.efiAdding boot menu entry for UEFI Firmware Settings ...done

When I tried to verify:

sudo dpkg --list | grep linux-image

I found:

ii linux-image-6.5.0-14-generic 6.5.0-14.14~22.04.1 amd64 Signed kernel image generic

ii linux-image-6.5.0-44-generic 6.5.0-44.44~22.04.1 amd64 Signed kernel image generic

ii linux-image-unsigned-6.2.0-060200-generic 6.2.0-060200.202302191831 amd64 Linux kernel image for version 6.2.0 on 64 bit x86 SMP

I wonder why this 6.2.0 is "unsigned"? It seems to have some influences because I cannot set it as the default kernel by changing the /etc/default/grub file.