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/build/dotenv/test.env

@@ -1,4 +1,4 @@
-TEST_DATABASE_MYSQL_DSN=mmuser:mostest@tcp(mysql:3306)/mattermost_test?charset=utf8mb4,utf8&readTimeout=30s&writeTimeout=30s&multiStatements=true&maxAllowedPacket=4194304
+TEST_DATABASE_MYSQL_DSN=mmuser:mostest@tcp(mysql:3306)/mattermost_test?charset=utf8mb4&readTimeout=30s&writeTimeout=30s&multiStatements=true&maxAllowedPacket=4194304
 TEST_DATABASE_POSTGRESQL_DSN=postgres://mmuser:mostest@postgres:5432/mattermost_test?sslmode=disable&connect_timeout=10
 TEST_DATABASE_MYSQL_ROOT_PASSWD=mostest
 GOBIN=/mattermost/server/bin