use newer ubuntu for old varnish

Author: dbu

.github/workflows/ci.yml

@@ -95,7 +95,7 @@ jobs:
 
   varnish5:
     name: PHP ${{ matrix.php }} Legacy Varnish 5
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       VARNISH_VERSION: '5.1'
       VARNISH_MODULES_VERSION: '0.14.0'
@@ -132,7 +132,7 @@ jobs:
 
   varnish4:
     name: PHP ${{ matrix.php }} Legacy Varnish 4
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       VARNISH_VERSION: '4.1'
       VARNISH_MODULES_VERSION: '' # varnish modules compilation fails, not sure why