IPv6 support

[marek22k]

IPv4 is a legacy protocol and should only be used for backward compatibility nowadays.
IPv6 should be added in the following places:

1. For crawlers: On https://about.marginalia-search.com/article/crawler/ and the crawler's ability to index IPv6-only pages
   Otherwise, there could even be problems with websites in PR Add my homepage and a few other websites I link to submit-site-to-marginalia-search#472.
2. For the search engine itself: The domain marginalia-search.com only has an IPv4 address

[vlofgren]

Crawler should support IPv6, though the server deployment isn't configured for it. Will migrate the deployment off docker sometime in April, and re-do most of the networking as part of that. Should be a good opportunity to address the IPv6 configuration.

Closing the issue since it's not really related to the source code of the search engine.