### What
Remove the auto appending `.xml` extension to the sitemap routes when
it's a dynamic route.
### Why
Previously we were adding `.xml` to `/[...paths/]sitemap` routes, but
the bad part is when you use it to generate multiple sitemaps with
`generateSitemaps` in format like `/[...paths/]sitemap.xml/[id]`, which
doesn't look good in url format and it can be inferred as xml with
content-type. Hence we don't need to add `.xml` in the url.
Before this change it could also result into the different url between
dev and prod:
dev: `/sitemap.xml/[id]`
prod: `/sitemap/[id].xml`
Now it's going to be aligned as `/sitemap/[id]`. Users can add extension
flexiblely.
Closes NEXT-3357
09baadfd70