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
```

store/sqlstore/plugin_store.go

@@ -11,8 +11,8 @@ import (
 	sq "github.com/Masterminds/squirrel"
 	"github.com/pkg/errors"
 
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/store"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/store"
 )
 
 const (