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:

• 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 :warning:
qgis-app/plugins/tests/test_validator.py	33.33%	2 Missing :warning:
qgis-app/plugins/forms.py	0.00%	1 Missing :warning:
qgis-app/plugins/validator.py	50.00%	1 Missing :warning:

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Additional details and impacted files

```diff @@ 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.