GitLabCI: remove old mono version pipeline

Because it is red (because nuget doesn't accept old TLS connections,
and only newer versions of Mono support newer TLS versions)

.gitlab-ci.yml

@@ -11,18 +11,6 @@ stages:
   - build
 
 stockmono_build:
-  image: ubuntu:18.04
-  stage: build
-  script:
-    # https://askubuntu.com/a/1013396
-    - DEBIAN_FRONTEND=noninteractive apt install -y mono-complete mono-xbuild fsharp
-    - mono --version
-
-    - curl -o nuget.exe $NUGET_451_URL
-    - mono nuget.exe restore src/TgSharp.sln
-    - xbuild src/TgSharp.Core/TgSharp.Core.csproj
-
-stocknewmono_build:
   image: ubuntu:20.04
   stage: build
   script:
@@ -35,7 +23,7 @@ stocknewmono_build:
     - xbuild src/TgSharp.Core/TgSharp.Core.csproj
 
 newmono_build:
-  image: ubuntu:18.04
+  image: ubuntu:20.04
   stage: build
   artifacts:
     paths:

scripts/install_mono_from_microsoft_deb_packages.sh

@@ -1,6 +1,8 @@
 #!/usr/bin/env bash
 set -euxo pipefail
 
+source /etc/os-release
+
 # required by apt-key
 apt install -y gnupg2
 # required by apt-update when pulling from mono-project.com
@@ -8,8 +10,8 @@ apt install -y ca-certificates
 
 # taken from http://www.mono-project.com/download/stable/#download-lin
 apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
-# bionic(=18.04) below works even for 18.10, 19.04 and 19.10
-echo "deb https://download.mono-project.com/repo/ubuntu stable-bionic main" | tee /etc/apt/sources.list.d/mono-official-stable.list
+echo "deb https://download.mono-project.com/repo/ubuntu stable-$UBUNTU_CODENAME main" | tee /etc/apt/sources.list.d/mono-official-stable.list
+
 apt update
-apt install -y mono-devel
+DEBIAN_FRONTEND=noninteractive apt install -y mono-devel
 mono --version