github actions generates debian packages incompatibles with debian 11

[landryb]

since actions/runner-images#6399 using ubuntu-latest generates a debian package that debian 11 doesnt know what to do with:

$dpkg --info /tmp/georchestra-mapstore-f3ce6b9eb6f684bc0932b54af7738e965d8c26c2.deb 
dpkg-deb: error: archive '/tmp/georchestra-mapstore-f3ce6b9eb6f684bc0932b54af7738e965d8c26c2.deb' uses unknown compression for member 'control.tar.zst', giving up

using ubuntu-20.04 instead should fix that.

[landryb]

the deb artifact on https://github.com/georchestra/mapstore2-georchestra/actions/runs/3947504132 can be read by a debian 11 dpkg:

[18/01 10:47] root@demo.fluela:/tmp $dpkg --info georchestra-mapstore-ef8bc0153e6c95c6be9f8c660c6144a3e5b10e33.deb 
 new Debian package, version 2.0.
 size 120847692 bytes: control archive=1076 bytes.
     673 bytes,    14 lines      conffiles            
     259 bytes,     9 lines      control              
    1406 bytes,    17 lines      md5sums              
 Package: georchestra-mapstore
 Version: 9999.master.202301180942~ef8bc01

will cherrypick ef8bc01 on 2022.02.xx branch.