ci: drop Ubuntu 20.04 support (#6630)

.CI/CreateUbuntuDeb.sh

@@ -20,10 +20,6 @@ deb_path="Chatterino-ubuntu-${ubuntu_release}-x86_64.deb"
 
 # Refactor opportunity:
 case "$ubuntu_release" in
-    20.04)
-        # Qt6 static-linked deb, see https://github.com/Chatterino/docker
-        dependencies="libc6, libstdc++6, libblkid1, libbsd0, libexpat1, libffi7, libfontconfig1, libfreetype6, libglib2.0-0, libglvnd0, libglx0, libgraphite2-3, libharfbuzz0b, libicu66, libjpeg-turbo8, libmount1, libnotify4, libopengl0, libpcre2-16-0, libpcre3, libpng16-16, libselinux1, libssl1.1, libuuid1, libx11-xcb1, libxau6, libxcb1, libxcb-cursor0, libxcb-glx0, libxcb-icccm4, libxcb-image0, libxcb-keysyms1, libxcb-randr0, libxcb-render0, libxcb-render-util0, libxcb-shape0, libxcb-shm0, libxcb-sync1, libxcb-util1, libxcb-xfixes0, libxcb-xkb1, libxdmcp6, libxkbcommon0, libxkbcommon-x11-0, zlib1g"
-        ;;
     22.04)
         # Qt6 static-linked deb, see https://github.com/Chatterino/docker
         dependencies="libc6, libstdc++6, libglx0, libopengl0, libpng16-16, libharfbuzz0b, libfreetype6, libfontconfig1, libjpeg-turbo8, libxcb-glx0, libegl1, libx11-6, libxkbcommon0, libx11-xcb1, libxkbcommon-x11-0, libxcb-cursor0, libxcb-icccm4, libxcb-image0, libxcb-keysyms1, libxcb-randr0, libxcb-render-util0, libxcb-shm0, libxcb-sync1, libxcb-xfixes0, libxcb-render0, libxcb-shape0, libxcb-xkb1, libxcb1, libbrotli1, libglib2.0-0, zlib1g, libicu70, libpcre2-16-0, libssl3, libgraphite2-3, libexpat1, libuuid1, libxcb-util1, libxau6, libxdmcp6, libffi8, libmount1, libnotify4, libselinux1, libpcre3, libbsd0, libblkid1, libpcre2-8-0, libmd0"

.CI/full-ubuntu-build.sh

@@ -10,12 +10,6 @@ env
 BUILD_TESTS="On"
 BUILD_BENCHMARKS="ON"
 
-ubuntu_version="$(lsb_release -sr)"
-if [ "$ubuntu_version" = "20.04" ]; then
-    BUILD_TESTS="Off"
-    BUILD_BENCHMARKS="Off"
-fi
-
 rm -rf build
 mkdir build
 cmake \