fix CI tests to ubuntu-20.04 because ubuntu-22.04 (current -latest) doesn't support MySQL 5.7

Author: dm-2

.github/workflows/ci.yml

@@ -5,7 +5,7 @@ on: [pull_request]
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2

.github/workflows/replica-tests.yml

@@ -5,7 +5,7 @@ on: [pull_request]
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         version: [mysql-5.7.25,mysql-8.0.16]