add percona to versions in workflows

Author: lukelewang

.github/workflows/replica-tests.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        version: [mysql-5.7.25,mysql-8.0.16]
+        version: [mysql-5.7.25,mysql-8.0.16,PerconaServer-8.0.21]
 
     steps:
     - uses: actions/checkout@v2