Use HTTPS for external links from XML files where possible

[spring-operator]

This commit updates URLs to prefer the https protocol. Redirects are not followed to avoid accidentally expanding intentionally shortened URLs (i.e. if using a URL shortener).

HTTP URLs that Could Not Be Fixed

These URLs were unable to be fixed. Please review them to see if they can be manually resolved.

• http://xslthl.sf.net (301) with 1 occurrences could not be migrated:
  (https result AnnotatedConnectException).

Fixed URLs

Fixed Success

These URLs were switched to an https URL with a 2xx status. While the status was successful, your review is still recommended.

• http://asciidoctor.org with 1 occurrences migrated to:
  https://asciidoctor.org (https result 200).
• http://sourceforge.net/projects/xslthl/ with 14 occurrences migrated to:
  https://sourceforge.net/projects/xslthl/ (https result 200).
• http://www.w3.org/TR/CSS21/propidx.html with 1 occurrences migrated to:
  https://www.w3.org/TR/CSS21/propidx.html (https result 200).

Ignored

These URLs were intentionally ignored.

• http://maven.apache.org/POM/4.0.0 with 8 occurrences
• http://www.w3.org/2001/XMLSchema-instance with 4 occurrences