Add clang21 devcontainer #134

Job	Run time
Windows (windows-2022, msvc)	2m 17s
Windows (windows-2019, msvc)	1m 59s
Linux (arbmind/cmake-gcc:3.25.1-12, gcc)	1m 29s
Linux (arbmind/cmake-clang:3.25.1-14, clang)	1m 30s
Linux (arbmind/cmake-gcc:3.27.6-13, gcc)	1m 14s
Linux (arbmind/cmake-clang:3.27.6-17, clang)	1m 25s
Linux (arbmind/cmake-clang-libstdcpp:3.25.1-15-12, clang-libstdcpp)	1m 27s
Linux (arbmind/cmake-gcc:3.25.1-11, gcc)	1m 10s
	12m 31s

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