Removes gcc-10 from the list of supported compilers. #513

Job	Run time
B2 msvc-14.2	5m 4s
CMake msvc-14.2 Debug C++17	8m 24s
B2 msvc-14.3	5m 49s
CMake msvc-14.2 Release C++17	7m 34s
CMake gcc-11 17 Debug	10m 49s
CMake gcc-11 20 Release	13m 19s
B2 gcc-11	3m 42s
CMake msvc-14.3 Debug C++20	7m 30s
CMake msvc-14.3 Release C++20	8m 22s
B2 clang-14	3m 45s
CMake clang-11 17 Debug	7m 18s
CMake clang-11 20 Debug	8m 23s
CMake clang-13 17 Release	10m 52s
CMake clang-13 20 Release	11m 6s
CMake clang-14 17 Debug -stdlib=libc++	9m 37s
CMake clang-14 20 Release -stdlib=libc++	13m 40s
	2h 15m 14s

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