[WIP] v2.x - Nokogiri Refactor Part 6 - Conversion of SignedDocument and final removal of REXML 🎉 #619

Job	Run time
rubocop	39s
Unit test (ubuntu-22.04, 3.4)	3m 47s
Unit test (ubuntu-22.04, jruby-9.4)	2m 42s
Unit test (ubuntu-22.04, 3)	3m 47s
Unit test (ubuntu-22.04, 3.2)	3m 49s
Unit test (macos-latest, 3)	1m 49s
Unit test (ubuntu-22.04, 3.3)	3m 40s
Unit test (ubuntu-22.04, 3.1)	3m 32s
Unit test (ubuntu-22.04, truffleruby)	5m 30s
Unit test (macos-latest, 3.2)	1m 44s
Unit test (macos-latest, jruby-9.4)	2m 28s
Unit test (windows-latest, 3.4)	3m 18s
Unit test (windows-latest, 3)	3m 20s
Unit test (macos-latest, 3.3)	2m 3s
Unit test (windows-latest, 3.3)	3m 18s
Unit test (macos-latest, 3.4)	1m 52s
Unit test (windows-latest, 3.1)	3m 7s
Unit test (macos-latest, 3.1)	2m 2s
Unit test (windows-latest, 3.2)	5m 4s
Unit test (macos-latest, truffleruby)	3m 37s
finish	5s
	1h 1m 13s

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