Fix sitemap (#22272)
Fix #22270.
Related to #18407.
The old code treated both sitemap and sitemap index as the format like:
```xml
...
<url>
<loc>http://localhost:3000/explore/users/sitemap-1.xml</loc>
</url>
...
```
Actually, it's incorrect for sitemap index, it should be:
```xml
...
<sitemap>
<loc>http://localhost:3000/explore/users/sitemap-1.xml</loc>
</sitemap>
...
```
See https://www.sitemaps.org/protocol.html
Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: delvh <dev.lh@web.de>
d34f3a2213