Update runners to ubuntu-24.04 from deprecated ubuntu-20.04 label

Author: billnapier

.github/workflows/system-tests-spanner-emulator.yaml

@@ -14,7 +14,7 @@ permissions:
   contents: read
 jobs:
   test:
-    runs-on: ubuntu-20.04
+    runs-on: 'ubuntu-24.04'
 
     services:
       emulator: