Use SPDX syntax for license expression

[dtolnay]

The license is supposed to be an SPDX 2.3 license expression. See https://doc.rust-lang.org/1.75.0/cargo/reference/manifest.html#the-license-and-license-file-fields.

SPDX license expressions support AND and OR operators to combine multiple licenses.

Previously multiple licenses could be separated with a /, but that usage is deprecated.