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/migration.env

@@ -6,11 +6,11 @@ MYSQL_USERNAME=root
 MYSQL_PASSWORD=mostest
 MYSQL_SCHEMA=mattermost_test
 POSTGRES_MIGRATIONS_PATH=./channels/db/migrations/postgres
-MYSQL_DSN=${MYSQL_USERNAME}:${MYSQL_PASSWORD}@tcp(mysql:3306)/${MYSQL_SCHEMA}?charset=utf8mb4,utf8&readTimeout=30s&writeTimeout=30s&multiStatements=true
+MYSQL_DSN=${MYSQL_USERNAME}:${MYSQL_PASSWORD}@tcp(mysql:3306)/${MYSQL_SCHEMA}?charset=utf8mb4&readTimeout=30s&writeTimeout=30s&multiStatements=true
 POSTGRES_DSN=postgres://${PG_USERNAME}:${PG_PASSWORD}@postgres:5432/${PG_SCHEMA}?sslmode=disable
 
 MM_SERVICESETTINGS_ENABLELOCALMODE=true
 MM_SQLSETTINGS_DRIVERNAME=mysql
-MM_SQLSETTINGS_DATASOURCE=mmuser:mostest@tcp(mysql:3306)/mattermost_test?charset=utf8mb4,utf8&readTimeout=30s&writeTimeout=30s&multiStatements=true&maxAllowedPacket=4194304
+MM_SQLSETTINGS_DATASOURCE=mmuser:mostest@tcp(mysql:3306)/mattermost_test?charset=utf8mb4&readTimeout=30s&writeTimeout=30s&multiStatements=true&maxAllowedPacket=4194304
 MAX_WAIT_SECONDS=500
 IS_CI=true