SPDX License Expression `LGPL-2.1` is deprecated

[Skgland]

I noticed while checking a project with cargo-deny that the spdx license expression used by this crate in Cargo.toml is deprecated.

See https://spdx.org/licenses/LGPL-2.1.html

LGPL-2.1-only, or LGPL-2.1-or-later should be used instead.