Update ci.yml

Author: sdrapkin

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
   build:
     strategy:
       matrix:
-        os: [ubuntu-latest, windows-latest]
+        os: [ubuntu-20.04, windows-latest]
     runs-on: ${{ matrix.os }}
     env:
       DOTNET_CLI_TELEMETRY_OPTOUT: true
@@ -26,6 +26,7 @@ jobs:
         uses: actions/setup-dotnet@v4
         with:
           dotnet-version: |
+            5.0.x
             6.0.x
             7.0.x
             8.0.x