Merge branch 'add_ubuntu_2004' into 'master'

Add build server for Ubuntu 20.04

See merge request !2077

.gitlab-ci.yml

@@ -42,6 +42,11 @@ build_ubuntu1804-gcc8:
    tags:
       - ubuntu1804-gcc8
 
+build_ubuntu2004:
+   extends: .build_template_linux
+   tags:
+      - ubuntu2004
+
 build_vc191x:
    extends: .build_template
    tags:

doc/software-manual.tex

@@ -258,7 +258,7 @@ be available in older compilers.
  \thead{Versions} \\
 \hline
 MS Visual Studio  & 2017 and 2019 \\
-GCC               & 7.3 and 8.3\\
+GCC               & 7.3, 8.3 and 9.3\\
 Xcode/clang       & latest \\
 \hline
 \end{tabular}