Document MSRV and test it in CI

[CraftSpider]

Currently, tectonic has no formal MSRV policy. However, conventionally, it attempts to avoid using features stabilized too recently - at least N-3 or so. It would be relatively easy to document the current minimum viable version in the README, and create a CI job testing it. From there, any change that bumps MSRV would be caught be CI, and require an explicit decision to increase.

This is not to say we should be particularly hesitant to do so, this would simply avoid issues like #1279 where very new features weren't noticed until after merging.