Update linting to the latest spdx-license list.

[dbuenzli]

I'm currently getting:

Warnings.
           warning 62: License doesn't adhere to the SPDX standard, see https://spdx.org/licenses/ : "Unicode-3.0"

> opam --version
2.3.0

[kit-ty-kate]

Thanks for the report. I've released spdx_licenses 1.3.0 and opened #6370 to fix this

[dbuenzli]

Thanks!

released spdx_licenses 1.3.0

Btw. I'd suggest to use the spdx version you support for your own versioning scheme. It makes things easier to comprehend (e.g. I did that in the Unicode libraries and I think it's nicer for end users).