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
```

api4/ldap_test.go

@@ -8,9 +8,9 @@ import (
 
 	"github.com/stretchr/testify/require"
 
-	"github.com/mattermost/mattermost-server/v5/einterfaces/mocks"
-	"github.com/mattermost/mattermost-server/v5/model"
-	"github.com/mattermost/mattermost-server/v5/plugin/plugintest/mock"
+	"github.com/mattermost/mattermost-server/v6/einterfaces/mocks"
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/plugin/plugintest/mock"
 )
 
 var spPrivateKey = `-----BEGIN PRIVATE KEY-----