Support for xz translations

- In the case of bullseye-security distributed by the debian security camp,
  the extension of Translation-en file of bullseye-security is xz format.
  (http://security.debian.org/debian-security/dists/bullseye-security/main/i18n/Translation-en.xz)
- Accordingly, the apt-mirror script was modified to support the xz extension.

Origin: upstream, https://github.com/apt-mirror/apt-mirror/pull/158


Closes: #932112, #932379
Signed-off-by: Benjamin Drung <benjamin.drung@canonical.com>