Deprecated license identifier "GPL-3.0"

See https://spdx.org/licenses/GPL-3.0.html
See https://spdx.org/licenses/

When running `cargo deny check`:
```
warning[parse-error]: error parsing SPDX license expression
   ┌─ /home/jens/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/subxt-0.42.1/Cargo.toml:34:26
   │
34 │ license = "Apache-2.0 OR GPL-3.0"
   │                          ─────── a deprecated license identifier was used
```