GitLab

* [ci] [C++] drop support for Visual Studio 2015 (fixes #6803)

* update more references

* [ci] some additional changes to drop VS2015 and make it work from solution file (#7046)

* Update LightGBM.vcxproj

* Update LightGBM.vcxproj.filters

* Update Installation-Guide.rst

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Co-authored-by: Nikita Titov <nekit94-08@mail.ru>