qBittorrent to drop Ubuntu 20.04 support

[brogers5]

Noticed this tidbit in the latest qBittorrent version's (4.3.5) release notes:

This is the last release in the 4.3.x series. The next series will probably drop support for Ubuntu 20.04 (Focal Fossa) because the Qt5 version is too old now.

As this image is currently based off Ubuntu 20.04, this may become problematic with future releases. Consider rebasing from a later version in the next release.

[chrisjohnson00]

Based on the discussion in https://github.com/qbittorrent/qBittorrent/issues/14452 Qt6 upgrade may not happen too soon. If 21.04 is supported, moving to that should be relatively trivial from the perspective of this container