Update license field following SPDX 2.1 license expression standard

[frisoft]

The new recommendation is to follow the SPDX 2.1 standard. This allows automatic license verification software to work properly. Reference: https://doc.rust-lang.org/cargo/reference/manifest.html#the-license-and-license-file-fields

[codecov]

Codecov Report

Merging #3209 (ea16270) into main (a3b4654) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #3209   +/-   ##
=======================================
  Coverage   50.42%   50.42%           
=======================================
  Files         436      436           
  Lines       42549    42549           
=======================================
  Hits        21457    21457           
  Misses      21092    21092           

[HalidOdat]

Thanks for the contribution! Looks good to me! :)