Add focal to conf_distributions

[orndorffgrant]

I haven't done a single thing to test this 🙃 but I was just trying to install the desktop app on Ubuntu 20.04 and it failed because there was no release for focal.

grant@arrakis ~> sudo apt update

Ign:5 https://packages.riot.im/debian focal InRelease
Err:6 https://packages.riot.im/debian focal Release
  404  Not Found [IP: 104.26.5.138 443]
Reading package lists... Done
E: The repository 'https://packages.riot.im/debian focal Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

I'm not sure if there is anything else that needs to be done to support this, but thought I'd throw this PR up to get it on your radar.

Signed-off-by: Grant Orndorff [email protected]

[jryans]

Thanks for the PR!

We've actually decided to switch to a general default distribution instead (element-hq/element-web#13367). It should appear with the riot-desktop package later today, and the desktop download instructions will be updated to match.