Move 2.1 requirements into subfolder; fix edDraftURIs

[kfranqueiro]

This does the following:

• Relocates the HTML file for WCAG 2.1's requirements under a subdirectory, consistent with 2.2's
  • This is not currently referenced by any actively-used build/deploy processes (republishing 2.1 docs is a future task)
  • This would technically fix one of the XSLT build targets, which assumes there's always a version subdirectory; however, that target has not been recently used and is slated for removal in Remove superseded XSLT build targets and update README #3985
• Updates edDraftURI in both the 2.1 and 2.2 files, as both referenced a nonexistent URL
  • You may notice that the editors' draft locations are inconsistent between 2.1 and 2.2; I am not planning on changing that (on the gh-pages branch) since the current locations are actively linked from each version of the recommendation, and it seems unlikely that the 2.1 rec would be updated, which would necessitate a long-standing redirect page.

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[w3cbot]

kfranqueiro marked as non substantive for IPR from ash-nazg.