Bug#924881: Suggested fix works

[Liam Morland <liam@morland.ca>]

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.