You have to manually edit /etc/apt/sources.list and possibly some files under /etc/apt/sources.list.d/ to not contain words focal or focal-*, but contain respective bionic and bionic-*, instead. (Only one of them per line.)
To solve the following error:
dpkg: Error processing mysql-server-8.0 (--configure) package:
the thread installed mysql-server-8.0 script post-installation package
returned error status 1
There were errors in processing:
E: Sub-process /usr/bin/dpkg returned an error code (1)
Create a /var/lib/dpkg/info/mysql-server-8.0.postrm with the ...