Remove all remaining unused XML/XSLT

[kfranqueiro]

This is a spiritual successor to #3985. It removes:

• The remainder of the XSLT build system, which has been unused since Implement Eleventy-based build system #3917 was merged 18 months ago
• The contents of lib, which were only necessary for running the XSLT build
• The entirety of the wcag20 folder, which has not been built in any way for an even longer time than that, and has typically only tripped up folks in the WCAG 2.x Backlog TF
• A reference to one wcag20 file in 11ty/biblio.ts, which has been replaced with a map directly containing the output that was previously computed at build time
• References to these files in the README, .gitignore, and the check-files workflow
  • Note this means that anyone who has ever run the XSLT build system locally (probably a low-single-digit number of people) will need to remove some old build artifacts that will show up in git status after pulling once this is merged

This branch occupies roughly 90MB less space than main when checked out, largely due to removal of lib and Flash and Silverlight examples under wcag20/Techniques/working-examples. (These will still exist in git history.)

This PR produces no changes in the Eleventy build output.

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[patrickhlauke]

this is magical! thanks for diving in on this long-standing bugbear of mine...

[patrickhlauke]

So stoked to see this merged!