doc: update gitlab links in sources and documentation files

.github/pull_request_template.md

CMake/FindEXPAT.cmake

CMake/FindFreetype.cmake

CMake/FindLibXml2.cmake

CMake/patches/99/FindHDF5.cmake

CMake/patches/99/FindOpenGL.cmake

CMake/setup.py.in

CONTRIBUTING.md

Common/DataModel/Testing/Cxx/TestPolyhedron2.cxx

Common/DataModel/Testing/Cxx/TestPolyhedron3.cxx

...on/DataModel/Testing/Cxx/UnitTestTriangleIntersection.cxx

Common/Misc/Testing/Cxx/TestPolygonBuilder5.cxx

Common/Misc/vtkContourValues.cxx

Documentation/dev/build_windows_vs.md

Documentation/dev/git/README.md

Documentation/dev/git/dashboard.md

Documentation/dev/git/develop.md

Documentation/maint/gitlab.md

Filters/Core/vtkImplicitProjectOnPlaneDistance.h

GUISupport/Qt/QVTKOpenGLStereoWidget.cxx