Bump up the minimum required MySQL version (#25465)

MySQL 5.7 is at end of life.

https://mattermost.atlassian.net/browse/MM-55589

```release-note
We bump up minimum MySQL version to be 8.0.0
```


Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

server/tests/test-config.json

@@ -90,7 +90,7 @@
     },
     "SqlSettings": {
         "DriverName": "mysql",
-        "DataSource": "mmuser:mostest@tcp(localhost:3306)/mattermost_test?charset=utf8mb4,utf8\u0026readTimeout=30s\u0026writeTimeout=30s\u0026maxAllowedPacket=4194304",
+        "DataSource": "mmuser:mostest@tcp(localhost:3306)/mattermost_test?charset=utf8mb4\u0026readTimeout=30s\u0026writeTimeout=30s\u0026maxAllowedPacket=4194304",
         "DataSourceReplicas": [],
         "DataSourceSearchReplicas": [],
         "MaxIdleConns": 20,