Bump to the minimum supported Rust version to 1.80.0

Obsidian-export now uses [std::sync::LazyLock](https://doc.rust-lang.org/std/sync/struct.LazyLock.html) instead of [lazy_static](https://crates.io/crates/lazy_static), which was only stabilized in Rust 1.80.0.
This change made it possible to drop the external dependency on lazy_static, though as a result of this, compiling with older versions will no longer be possible.