ci: replace removed Ubuntu 18.04 action runner

Author: gjasny

.github/workflows/cmake-ci.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [macOS-latest, ubuntu-18.04, windows-2019]
+        os: [macOS-latest, ubuntu-20.04, windows-2019]
         dependencies: [submodule, vcpkg]
     steps:
       - name: Checkout source