Lift MSRV requirement

[flokli]

As could be seen in the previous PRs, the MSRV requirement is causing a lot of churn:

• some crates are not available, or much worse than their current version
• the UX to edit Cargo.toml "to downgrade sufficiently" is terrible
• for an external contributor, this is only visible in CI or by building in some oldstable container locally

I understand you need to still ship an nsncd binary to Debian oldstable, but this is causing a lot of pain.

Could you use a more recent rust compiler to produce it?
Or branch off the Debian 10 bits to a "maintenance branch" and only cherry-pick necessary fixes to there?

[leifwalsh]

@geofft what do you think? I think you're the only one of us qualified to speak to whether we could do this.