the license you have in the cargo.toml points to GPL-3.0 which is a deprecated format. While this is usually interpreted as deprecated in favor of GPL-3.0-only in practice people have used to refer more broadly for GPLv3 (either one).
The REAMDE text clarifies that it is GPL-3.0-or-later , so the Cargo.toml should reflect that as well
the license you have in the cargo.toml points to GPL-3.0 which is a deprecated format. While this is usually interpreted as deprecated in favor of GPL-3.0-only in practice people have used to refer more broadly for GPLv3 (either one).
The REAMDE text clarifies that it is
GPL-3.0-or-later, so the Cargo.toml should reflect that as well