Move apache2 service to systemd in Focal

[emkll]

Description

In the https://github.com/freedomofpress/securedrop/blob/1d50b6ecd4bc0cfe6c803630b7fb4f8d39bc3810/install_files/securedrop-app-code/debian/postinst, we stop and restart the apache2 services during a securedrop-app-code package install. We should instead use systemd for Focal, like all the other services that are being run on the server.

[emkll]

Took a look at the files provided by the apache2 package in Focal, and systemd-specific services are in /lib and thus not active. However, they provide a sysv service in init.d

apache2: /lib/systemd/system/apache-htcacheclean.service
apache2: /lib/systemd/system/apache-htcacheclean@.service
apache2: /lib/systemd/system/apache2.service
apache2: /lib/systemd/system/apache2@.service
apache2: /etc/init.d/apache-htcacheclean
apache2: /etc/init.d/apache2

During review of #5638, @kushaldas stated that the systemd-sysv-generator [1] handled the service from the init script, providing backwards compatibility when using systemd.

Based on the above, I think we should close, as this is what is handled by the upstream package, and would cause unnecessary code to maintain.

[1] https://www.freedesktop.org/software/systemd/man/systemd-sysv-generator.html