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
```

einterfaces/mocks/DataRetentionJobInterface.go

@@ -5,7 +5,7 @@
 package mocks
 
 import (
-	model "github.com/mattermost/mattermost-server/v5/model"
+	model "github.com/mattermost/mattermost-server/v6/model"
 	mock "github.com/stretchr/testify/mock"
 )