HTTPS XML namespaces are invalid

[alexduryee]

While testing EAD exports on sandbox, I started getting invalid namespace errors. I dug into it a bit, and found that xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance" is not a valid namespace - the declaration requires http. It appears that namespace URIs are moreso unique identifiers than dereferencable URLs (ref. https://stackoverflow.com/a/75618537)

I haven't tested all of the serializers in #3106, but given that at least the XSI namespace was impacted, it would merit checking if various XML documents still validate.