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
```

plugin/api_timer_layer_generated.go

@@ -11,8 +11,8 @@ import (
 	"net/http"
 	timePkg "time"
 
-	"github.com/mattermost/mattermost-server/v5/einterfaces"
-	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v6/einterfaces"
+	"github.com/mattermost/mattermost-server/v6/model"
 )
 
 type apiTimerLayer struct {