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
```

web/main_test.go

@@ -6,8 +6,8 @@ package web
 import (
 	"testing"
 
-	"github.com/mattermost/mattermost-server/v5/shared/mlog"
-	"github.com/mattermost/mattermost-server/v5/testlib"
+	"github.com/mattermost/mattermost-server/v6/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/testlib"
 )
 
 var mainHelper *testlib.MainHelper