Increase our MSRV to 1.70

Nick Mathewsonrequested to merge
nickm/arti:msrv-bump-1.70 into main

This will allow us to upgrade to hickory (!1770 (closed), #1057 (closed)) and a few other things.

There are numerous "MSRV" comments in our code base about opportunities to clean things up when our version is higher.; we can update them at our leisure

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