Copy LICENCE file from root directory to plugin package

[jmkerloch]

We have some issue when using latest version (qgis-plugin-ci-2.8.1): https://gitlab.com/Oslandia/qgis/qwc2_tools/-/jobs/5598167137#L306

$ qgis-plugin-ci release ${CI_COMMIT_TAG} --allow-uncommitted-changes --osgeo-username $OSGEO_USER_NAME --osgeo-password $OSGEO_USER_PASSWORD
2023-11-22 17:24:14||ERROR||release||=== A fault occurred occurred ===
Fault code: 1
Fault string: <Fault 1: 'File upload must be a valid QGIS Python plugin compressed archive. Cannot find LICENSE in plugin package.'>

This seems to come from this commit of QGIS plugin repository : https://github.com/qgis/QGIS-Django/commit/91b9576a53c6b75693427bb2a1dc9a2730a8282f

# Checks for LICENCE file precense
    licensename = package_name + "/LICENSE"
    if licensename not in namelist:
        raise ValidationError(_("Cannot find LICENSE in plugin package."))

Do you think we should copy LICENCE file from root directory to plugin package ?

This will avoid a copy of the file in the CI.