We recommend that you update your robots.txt file to point to the HTTPS version of
your sitemap files. We also recommend listing only the HTTPS URLs in your sitemap.
Keep in mind that if you're redirecting each URL on your site from HTTP to HTTPS,
you will still have only one robots.txt file accessible to crawlers. For example,
if http://example.com/robots.txt redirects to
https://example.com/robots.txt, the contents of the HTTP version
won't be visible to Google and other search engines.