Alternate href urls breaks sitemap

Hello !

When i tried with alternate href like in the readme i have the following error on google console:
```
The element 'url' has invalid child element 'link' in namespace 'http://www.w3.org/TR/xhtml11/xhtml11_schema.html'. List of possible elements expected: any element in namespace 'http://www.google.com/schemas/sitemap-image/1.1, http://www.google.com/schemas/sitemap-mobile/1.0, http://www.google.com/schemas/sitemap-video/1.1'.
```

I think it is related to : https://developers.google.com/search/docs/specialty/international/localized-versions#example_2 . They said to Specify the xhtml namespace as follows:

xmlns:xhtml="http://www.w3.org/1999/xhtml"

I did not find a way to make it work with this plugin but maybe that's me who is wrong on my way to do it