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
```

services/cache/lru.go

@@ -11,7 +11,7 @@ import (
 	"github.com/tinylib/msgp/msgp"
 	"github.com/vmihailenco/msgpack/v5"
 
-	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v6/model"
 )
 
 // LRU is a thread-safe fixed size LRU cache.