[mythtvnz] Upgrade to Ubuntu 20.04

[Robert Fisher]

I just upgraded my Ubuntu Server to 20.04 via command line and SSH.

(Originally MythBuntu 16.04, then Ubuntu 18.04)

Only issues -

The process seemed to hang for 10 minutes after upgrading Chromium on 
SNAP but then it continued without prompting

MythWeb didn't work until I edited /etc/apache2/mods-enabled/php7.2.load 
to point to php7.4 (i then renamed it)

VNC didn't work because vnc4server is not in 20.4 so I changed to 
tightvncserver