Switch to `std::sync::LazyLock`

[Serial-ATA]

Usage of once_cell::sync::Lazy was replaced with std::sync::OnceLock awhile ago:

https://github.com/Serial-ATA/lofty-rs/blob/1505ddf69fd7df686bdf37e7306352011552c184/lofty/src/id3/v2/util/upgrade.rs#L41-L52

In 1.80.0, std::sync::LazyLock was finally stabilized. Now in that example, we can just go back to using V2_KEYS directly instead of having functions to access it.

This is blocked right now since 1.80.0 just came out and it really isn't that important to do anyway.

[Serial-ATA]

No longer blocked, new MSRV is 1.85