Revert "Do not gsub non ASCII characters in header anchor.".

This reverts commit 699babe.

Also change the upgrading_ruby_on_rails link back to original in 4_2_release_notes.

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I do not have good solution to take care all non-english substution :sweat: . I do not know if there are any translations out there depend on this code. So it's best revert to where it was. Then I [modify my translation's markdown.rb](https://github.com/docrails-tw/guides/commit/e0038a17e00ed1eccecc0a4a1d08b456cf2711d8) to take care my chinese translations.

Related: #16545 #16576

cc @chancancode