21 packages can be upgraded. Run 'apt list --upgradable' to see them.
:~$ sudo apt upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
The following packages have been kept back:
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
Why the 21 packages are not upgraded ?
What to do with kept back package ?