Bug#924881: Suggested fix works
I had the same problem trying to upgrade from postgresql 11 to 13. I got
the same "ee key too small" error.
pg_upgradecluster exited without creating a working upgrade and without
giving an error message. It did not appear to fail.
I dropped the non-working pg 13 cluster and then ran:
sudo make-ssl-cert generate-default-snakeoil --force-overwrite
Next, I ran pg_upgradecluster and it worked seemlessly.
pg_upgradecluster needs a more obvious error message when it is not able
to complete properly.