Choose and enforce as MSRV

[nyarly]

Describe the bug

The lorri project ought to choose, report and enforce an MSRV policy.

This is related both to the recent 1.80/time issue, and to lorri issues (and PRs!) related to Rust features (e.g. LazyLock.) A clear policy would make it easier for contributors to evaluate the urgency of their work, and for maintainers to decide when to accept it.

lorri builds and is backported onto the current versions of Nixpkgs, which implies an MSRV that it uses to be built. CI reflects this, so "whatever nixpkgs is using" is the de facto policy.

However, lorri development is done using Rustup, so the Rust version used there is decoupled from the deployed Rust version. Developers can maintain parity with care, though.

Additional context

Tooling around determining current versions of nixpkgs and NixOS, and the Rust toolchain available in each would be helpful. Also tooling to update documentation with versions easily.