Use SPDX license expression

[gerritsangel]

• [x] I agree to follow the project's code of conduct.

• [x] I added an entry to rstar/CHANGELOG.md if knowledge of this change could be valuable to users.

Uses a SPDX license expression as recommended by Cargo.

This enables e.g. parsing via the CycloneDX Rust Cargo Plugin.

[rmanoka]

bors r+

thanks @gerritsangel

[bors[bot]]

Build succeeded:

• ci result