Remove once_cell from jwt example

[Hmikihiro]

Motivation

I think using std::sync::LazyLock can reduce dependency once_cell.

[fr0staman]

std::sync::LazyLock introduced in 1.80.0, which axum MSRV with 0.8.0 is 1.75.0.

Maybe in future, but not right now, maybe

[ttys3]

since Rust it self growing fast, I think following newest Rust in main branch is good

[jplatte]

I think we already don't enforce an MSRV other than stable for examples. The CI failure looks unrelated.