ci: Switch from bionic to buster

This change is a prerequisite for the following bumping Qt minimum
version to 5.11.3. It is required as bionic has Qt 5.9.5.

Effectively, this also changes:
- gcc from 8.4.0 to 8.3.0
- python from 3.6.5 to 3.7.3

.cirrus.yml

@@ -195,7 +195,7 @@ task:
     FILE_ENV: "./ci/test/00_setup_env_i686_centos.sh"
 
 task:
-  name: '[previous releases, uses qt5 dev package and some depends packages, DEBUG] [unsigned char] [bionic]'
+  name: '[previous releases, uses qt5 dev package and some depends packages, DEBUG] [unsigned char] [buster]'
   previous_releases_cache:
     folder: "releases"
   << : *GLOBAL_TASK_TEMPLATE

ci/test/00_setup_env_native_qt5.sh

@@ -7,7 +7,7 @@
 export LC_ALL=C.UTF-8
 
 export CONTAINER_NAME=ci_native_qt5
-export DOCKER_NAME_TAG=ubuntu:18.04  # Check that bionic gcc-8 can compile our C++17 and run our functional tests in python3, see doc/dependencies.md
+export DOCKER_NAME_TAG=debian:buster  # Check that buster gcc-8 can compile our C++17 and run our functional tests in python3, see doc/dependencies.md
 export PACKAGES="gcc-8 g++-8 python3-zmq qtbase5-dev qttools5-dev-tools libdbus-1-dev libharfbuzz-dev"
 export DEP_OPTS="NO_QT=1 NO_UPNP=1 NO_NATPMP=1 DEBUG=1 ALLOW_HOST_PACKAGES=1 CC=gcc-8 CXX=g++-8"
 export TEST_RUNNER_EXTRA="--previous-releases --coverage --extended --exclude feature_dbcrash"  # Run extended tests so that coverage does not fail, but exclude the very slow dbcrash