ci: Remove automatic build support for Ubuntu 18.04

The majority of users should have already switched or upgraded to Ubuntu 20.04 at this point, so there is no point in building further Ubuntu 18.04 versions. This saves us some CI slots, and also reduces the number of target systems to actually support.

.github/workflows/main.yml

@@ -92,24 +92,9 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        ubuntu: [ bionic, focal ]
+        ubuntu: [ focal ]
         compiler: [ gcc, clang ]
         include:
-          - ubuntu: bionic
-            compiler: gcc
-            compiler-cxx: g++
-            runner: ubuntu-18.04
-            packages: gcc-8 g++8
-            extra_command: sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 800 --slave /usr/bin/g++ g++ /usr/bin/g++-8
-            id: ubuntu-18.04
-          - ubuntu: bionic
-            compiler: clang
-            compiler-cxx: clang++
-            compiler-version: 8
-            runner: ubuntu-18.04
-            packages: clang-8 clang-format-8 clang-tidy-8
-            extra_command: ""
-            id: ubuntu-18.04-clang
           - ubuntu: focal
             compiler: gcc
             compiler-cxx: g++