Update license field name according to PEP 639

[frostming]

Pull Request Check List

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

Describe what you have changed in this PR.

[codecov-commenter]

Codecov Report

Merging #959 (30fe869) into main (674ef99) will increase coverage by 0.00%.
The diff coverage is 72.72%.

@@           Coverage Diff           @@
##             main     #959   +/-   ##
=======================================
  Coverage   83.55%   83.55%           
=======================================
  Files          74       74           
  Lines        6471     6472    +1     
  Branches     1342     1342           
=======================================
+ Hits         5407     5408    +1     
  Misses        747      747           
  Partials      317      317           

Flag	Coverage Δ
unittests	83.31% <72.72%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pdm/formats/flit.py	75.55% <0.00%> (ø)
pdm/models/project_info.py	91.11% <33.33%> (-4.13%)	⬇️
pdm/cli/actions.py	79.88% <100.00%> (ø)
pdm/formats/legacy.py	76.92% <100.00%> (+1.45%)	⬆️
pdm/formats/poetry.py	91.79% <100.00%> (ø)

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