Removes gcc-10 from the list of supported compilers. #506

Job	Run time
B2 msvc-14.2	5m 30s
CMake gcc-11 17 Debug	10m 49s
B2 gcc-11	3m 39s
CMake gcc-11 20 Release	13m 14s
B2 clang-14	3m 42s
CMake clang-11 17 Debug	7m 6s
CMake msvc-14.2 Debug C++17	6m 14s
B2 msvc-14.3	6m 10s
CMake clang-11 20 Debug	7m 36s
CMake msvc-14.2 Release C++17	9m 23s
CMake clang-13 17 Release	10m 51s
CMake msvc-14.3 Debug C++20	6m 8s
CMake clang-13 20 Release	11m 6s
CMake msvc-14.3 Release C++20	8m 11s
CMake clang-14 17 Debug -stdlib=libc++	9m 43s
CMake clang-14 20 Release -stdlib=libc++	13m 25s
	2h 12m 47s

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