Revert "Use the ubuntu-20.04 runner as 18.04 is deprecated (rescript-lang#5626)"

This reverts commit 4c83939.

Author: cknitt

.github/workflows/ci.yml

@@ -11,9 +11,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        # Stay on the oldest Ubuntu version that's still supported by Github Actions
-        # to avoid glibc incompatibilities as far as possible.
-        os: [macos-latest, ubuntu-20.04, windows-latest, macos-arm]
+        # Use ubuntu-18.04 because of glibc issues,
+        # see https://github.com/rescript-lang/rescript-vscode/issues/216.
+        os: [macos-latest, ubuntu-18.04, windows-latest, macos-arm]
         ocaml_compiler: [4.14.0]
 
     runs-on: ${{matrix.os}}