MM-36271: Bump major version to 6.0 (#17973)

https://mattermost.atlassian.net/browse/MM-36271

```release-note
We bump the major version to 6.0
```

app/plugin_api_tests/test_db_driver/main.go

@@ -7,12 +7,12 @@ import (
 	"database/sql"
 	"testing"
 
-	"github.com/mattermost/mattermost-server/v5/app/plugin_api_tests"
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/plugin"
-	"github.com/mattermost/mattermost-server/v5/shared/driver"
-	"github.com/mattermost/mattermost-server/v5/store/sqlstore"
-	"github.com/mattermost/mattermost-server/v5/store/storetest"
+	"github.com/mattermost/mattermost-server/v6/app/plugin_api_tests"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/plugin"
+	"github.com/mattermost/mattermost-server/v6/shared/driver"
+	"github.com/mattermost/mattermost-server/v6/store/sqlstore"
+	"github.com/mattermost/mattermost-server/v6/store/storetest"
 )
 
 type MyPlugin struct {