ci: fix up unsupported host system version

.github/workflows/go.yml

@@ -176,7 +176,7 @@ jobs:
     strategy:
       matrix:
         go-version: [ 1.23.x ]
-        platform: [ ubuntu-18.04 ]
+        platform: [ ubuntu-20.04 ]
     runs-on: ${{ matrix.platform }}
     steps:
       - name: Start MySQL server