When trying to run do-release-upgrade, the process stops with …
Not enough free disk space
The upgrade has aborted. The upgrade needs a total of 55.8 M free
space on disk ‘/boot’. Please free at least an additional 47.5 M of
disk space on ‘/boot’. Empty your trash and remove temporary packages
of former installations using ‘sudo apt-get clean’.
A quick df will show that the /boot partition is indeed nearly full.
Filesystem 1K-blocks Used Available Use% Mounted on