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/channels/store/storetest/settings.go

@@ -22,10 +22,10 @@ import (
 )
 
 const (
-	defaultMysqlDSN        = "mmuser:mostest@tcp(localhost:3306)/mattermost_test?charset=utf8mb4,utf8&readTimeout=30s&writeTimeout=30s&multiStatements=true&maxAllowedPacket=4194304"
+	defaultMysqlDSN        = "mmuser:mostest@tcp(localhost:3306)/mattermost_test?charset=utf8mb4&readTimeout=30s&writeTimeout=30s&multiStatements=true&maxAllowedPacket=4194304"
 	defaultPostgresqlDSN   = "postgres://mmuser:mostest@localhost:5432/mattermost_test?sslmode=disable&connect_timeout=10"
 	defaultMysqlRootPWD    = "mostest"
-	defaultMysqlReplicaDSN = "root:mostest@tcp(localhost:3307)/mattermost_test?charset=utf8mb4,utf8\u0026readTimeout=30s"
+	defaultMysqlReplicaDSN = "root:mostest@tcp(localhost:3307)/mattermost_test?charset=utf8mb4\u0026readTimeout=30s"
 )
 
 func getEnv(name, defaultValue string) string {