[electrum] bump docker to ubuntu 20.04 for AppImage building

Summary:
We want to build the linux releases on an old linux version to produce an AppImage that works on the widest possible range of linux distros, but Ubuntu 18.04 is now too old for compatibility with various python packages.

For instance we need Ubuntu 20.04 because newer cryptography version (updated in a follow-up commit to bump the package python version) need a newer OpenSSL version than is available on Bionic.

see https://github.com/pyca/cryptography/pull/8449

This is a partial backport of electroncash#2774, electroncash#2778 and electroncash#2755

Test Plan:

cd electrum
contrib/make_clean
contrib/build-linux/appimage/build.sh

Then run the release binary: dist/ElectrumABC-5.2.11-x86_64.AppImage

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15522