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/scheduler/plugin.go

@@ -4,8 +4,8 @@
 package scheduler
 
 import (
-	"github.com/mattermost/mattermost-server/v5/app"
-	tjobs "github.com/mattermost/mattermost-server/v5/jobs/interfaces"
+	"github.com/mattermost/mattermost-server/v6/app"
+	tjobs "github.com/mattermost/mattermost-server/v6/jobs/interfaces"
 )
 
 type PluginsJobInterfaceImpl struct {