Drop about-to-be-removed Ubuntu 20.04 from CI (fixes #282) (#283)

* CI: Drop about-to-be-removed Ubuntu 20.04 CI

* CI: Add missing "sudo apt-get update"

Also add --no-install-recommends (to install as few as possible
additional packages) packages and use apt-get rather than
apt because the target is a machine (rather thamn a human).

.github/workflows/cmake-multi-platform.yml

diff --git a/.github/workflows/cmake-multi-platform.yml b/.github/workflows/cmake-multi-platform.yml
index 4d66b65..ab89c2c 100644
--- a/.github/workflows/cmake-multi-platform.yml
+++ b/.github/workflows/cmake-multi-platform.yml
@@ -40,16 +40,6 @@
             c_compiler: clang
             cpp_compiler: clang++
             bundled_protobuf: 'OFF'
-          - os: ubuntu-20.04
-            build_type: Release
-            c_compiler: clang-12
-            cpp_compiler: clang++-12
-            bundled_protobuf: 'OFF'
-          - os: ubuntu-20.04
-            build_type: Debug
-            c_compiler: clang-12
-            cpp_compiler: clang++-12
-            bundled_protobuf: 'OFF'
 
     steps:
     - uses: actions/checkout@v3
@@ -67,7 +57,8 @@
           )
         fi
         set -x
-        sudo apt install -y "${ubuntu_packages[@]}"
+        sudo apt-get update
+        sudo apt-get install --no-install-recommends -y "${ubuntu_packages[@]}"
 
     - name: Set reusable strings
       # Turn repeated input strings (such as the build output directory) into step outputs. These step outputs can be used throughout the workflow file.