community/msgsl: upgrade to 4.1.0

https://github.com/microsoft/GSL/releases/tag/v4.1.0

Temporarily build with -Wno-unused-value to avoid build error on 32-bit arm (more info). Error found the 3.21 armv7 builder (error log).

/home/buildozer/aports/community/msgsl/src/GSL-4.0.0/tests/span_tests.cpp: In member function 'virtual void span_test_std_container_ctad_Test::TestBody()':
/home/buildozer/aports/community/msgsl/src/GSL-4.0.0/tests/span_tests.cpp:1275:34: error: value computed is not used [-Werror=unused-value]
 1275 |         std::string_view sv{"foo"};
      |                                  ^
cc1plus: all warnings being treated as errors