Upgrade Linux distributions used on OTB CI
OTB is tested on CI on the following distributions :
- OSX Mojave
- Windows 8
- Windows 10
- Centos 6.6 (docker)
- Ubuntu 18.04 (docker)
- Debian Testing (docker)
We could upgrade the Centos and Ubuntu docker images to version 7 and 20.04 respectively.
Qt >= 5.12 is not compatible with CentOS 6.6 (see #2090).
Note that with redhat 7 (8?) we might run into performances issues, see #2065