Revert to ubuntu 18.04 in CI (#5871)

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

This reverts commit 4c83939.

* Update CHANGELOG

* Update comment

Author: cknitt

.github/workflows/ci.yml

@@ -11,9 +11,10 @@ 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 for glibc compatibility with older Linux distributions.
+        # Note that this will only be supported by Github until April 1st, 2023,
+        # see https://github.com/actions/runner-images/issues/6002.
+        os: [macos-latest, ubuntu-18.04, windows-latest, macos-arm]
         ocaml_compiler: [4.14.0]
 
     runs-on: ${{matrix.os}}

CHANGELOG.md

@@ -15,6 +15,7 @@
 #### :bug: Bug Fix
 
 - Prevent inlining of async functions in additional cases https://github.com/rescript-lang/rescript-compiler/issues/5860
+- Revert to ubuntu-18.04 in CI to support Linux versions with older glibc https://github.com/rescript-lang/rescript-compiler/issues/5868
 
 # 10.1.0