[LXC] ubuntu 20.10 EOL, upgrade to ubunut 21.10 [1]

[1] https://wiki.ubuntu.com/Releases

Tested by::

    # build the container ..
    $ sudo -H ./utils/lxc.sh build searx-ubu2110

    # open a shell in the container
    $ sudo -H ./utils/lxc.sh cmd searx-ubu2110 bash
    [root@searx-ubu2110 SearXNG]#

    # install a complete SearXNG suite ..
    [root@searx-ubu2110 SearXNG]# ./utils/searx.sh install all
    ...

    # install apache to export the SearXNG instance by HTTP
    [root@searx-ubu2110 SearXNG]# ./utils/searx.sh apache install
    ...
    INFO:  got 200 from http://10.174.184.94/searx



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>