Hi,
This command remove this packages for me : bind9-utils bind9utils dnsutils libicu63 libllvm7 libreadline7 php-gettext postgresql-11 postgresql-client-11 python3-ply python3-publicsuffix
It’s always cool, but not what we want 
I’ve tried to move some things of my boot folder : I’ve moved all files with 4.19.0-18 and 4.19.0-19.
On the safe side, I’ve only moved that in another folder rather than delete it. After that, I could do the migration !
I’ve read on the net that it’s better to keep one version before the current version. So all the others can be deleted.
After migration, I’ve that in /boot folder : (as you ca see, the migration deletes automatically the 4.19.0-20 files)
root@serveur:/boot# ls -l
total 87535
-rw-r--r-- 1 root root 206378 30 juin 14:52 config-4.19.0-21-amd64
-rw-r--r-- 1 root root 236286 13 août 15:25 config-5.10.0-17-amd64
drwxr-xr-x 5 root root 1024 22 août 15:04 grub
-rw-r--r-- 1 root root 35855210 22 août 10:58 initrd.img-4.19.0-21-amd64
-rw-r--r-- 1 root root 37640602 22 août 15:02 initrd.img-5.10.0-17-amd64
drwx------ 2 root root 12288 29 mai 2021 lost+found
-rw-r--r-- 1 root root 3417138 30 juin 14:52 System.map-4.19.0-21-amd64
-rw-r--r-- 1 root root 83 13 août 15:25 System.map-5.10.0-17-amd64
-rw-r--r-- 1 root root 5299456 30 juin 14:52 vmlinuz-4.19.0-21-amd64
-rw-r--r-- 1 root root 6962816 13 août 15:25 vmlinuz-5.10.0-17-amd64
I’ve reboot my server (systemctl reboot), all is fine !
And to finish, I’ve deleted the folder with moved files.