Making license file not required for now

[Xpirix]

According to the discussion and suggestions at https://lists.osgeo.org/pipermail/qgis-developer/2023-November/066275.html, this PR is a fix for the breaking change on the plugin upload/update workflow. This is about the following issue:

• Plugin licence #38

For now, a license file should be recommended. When missing, it should generate a non-blocking warning either for new or existing plugins.

Please find below a screenshot of the warning when uploading or updating a plugin package that doesn't contain a license file:

[codecov-commenter]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (08f01b4) 32.52% compared to head (24b468c) 32.51%.

Files	Patch %	Lines
qgis-app/plugins/views.py	0.00%	3 Missing ⚠️
qgis-app/plugins/tests/test_validator.py	33.33%	2 Missing ⚠️
qgis-app/plugins/forms.py	0.00%	1 Missing ⚠️
qgis-app/plugins/validator.py	50.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #314      +/-   ##
==========================================
- Coverage   32.52%   32.51%   -0.02%     
==========================================
  Files          71       71              
  Lines        4218     4217       -1     
==========================================
- Hits         1372     1371       -1     
  Misses       2846     2846              

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[Xpirix]

Please find below a screenshot of the updated warning according to this comment.