Update test.yml

Author: NielsdeBlaauw

.github/workflows/test.yml

@@ -8,7 +8,7 @@ jobs:
         php: ['7.4']
       fail-fast: false
     name: PHP ${{ matrix.php }}
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
     - name: Checkout repository
       uses: actions/checkout@v2