Revert "Update Ruby version and use latest artifact upload action"

This reverts commit 63119c3.

.github/workflows/lint-unit.yml

@@ -30,7 +30,7 @@ jobs:
       matrix:
         # we need to test the Ruby versions of the products we release RMT for,
         # those are: SLE15 (2.5.8, 2.5.9)
-        ruby-version: [ '2.5.9']
+        ruby-version: ['2.5.8', '2.5.9']
         # skipping Ruby3 tests for now, because RMT isn't Ruby3 compatible yet, and the failing 
         # tests confuse the team.
         # ruby-version: ['2.5.8', '2.5.9', '3.0.3']
@@ -92,10 +92,3 @@ jobs:
           echo "::group::Version verification checks"
           ruby ci/check-version-matches.rb
           echo "::endgroup::"
-
-      - name: Export coverage report
-        uses: actions/upload-artifact@v4
-        if: always()
-        with:
-          name: coverage
-          path: ${{ github.workspace }}/coverage