Update license
Since we no longer depend on any GPL library, we can switch to just LGPL.
Specifying a valid SPDX license string will also make it easier for distros to automatically check if the project is redistributable – OtherLicense
requires manual override for our build infrastructure: https://github.com/NixOS/nixpkgs/pull/78559#discussion_r371048384
I am not sure if we also do not need to bump cabal-version
property, as the SPDX identifiers support was supposed to be added in https://www.haskell.org/cabal/users-guide/developing-packages.html#pkg-field-license