Update Ruby version and use latest artifact upload action

.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: ['2.5.8', '2.5.9']
+        ruby-version: [ '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']
@@ -62,7 +62,7 @@ jobs:
           bundler-cache: true
 
       - name: Export coverage report
-        uses: actions/upload-artifact@v2
+        uses: actions/upload-artifact@v4
         with:
           name: coverage
           path: ${{ github.workspace }}/coverage