Remove all Focal-specific code from the codebase

[eloquence]

Soon after the 1 month grace period (from June 1-30), we should remove all Focal-specific code from the codebase, including any Focal-specific application code, config, provisioning logic, or developer tooling, and any Python 3.8-specific code, configuration or dependencies.

[mig5]

Regarding '1 month grace period', thought I would note that the Demo interfaces have just gone down, because https://demo-source.securedrop.org is throwing a 503, I believe due to the OS being past its EOL date. (It's a Focal image)

The Dockerfiles in https://github.com/freedomofpress/securedrop/blob/develop/securedrop/dockerfiles probably need to be updated to Noble.

[eloquence]

For the sprint kicking off today, as a lower priority goal, we're focusing on removing Focal-specific dependencies, with further cleanup planned after that.