Upgrade to go 1.21, replace golang.org/x/exp/slices with slices #1125

Job	Run time
build (macos-12, tool)	1m 47s
cpp-lib-build (macos-12, clang, ON)	1m 2s
cpp-lib-build (macos-12, clang, OFF)	1m 17s
build (macos-12, cpp)	14m 26s
build (macos-12, csharp)	8m 11s
cpp-lib-build (macos-12, gcc, ON)	1m 15s
build (macos-12, dart)	3m 47s
cpp-lib-build (macos-12, gcc, OFF)	1m 24s
build (macos-12, go)	1m 56s
build (macos-12, java)	1m 56s
cpp-lib-build (ubuntu-20.04, clang, ON)	1m 16s
build (macos-12, javascript)	2m 0s
cpp-lib-build (ubuntu-20.04, clang, OFF)	1m 0s
build (macos-12, typescript)	18m 48s
cpp-lib-build (ubuntu-20.04, gcc, ON)	1m 34s
build (macos-12, php)	2m 36s
cpp-lib-build (ubuntu-20.04, gcc, OFF)	55s
build (macos-12, python3)	1m 28s
cpp-lib-build (windows-2022, clang, ON)	4m 2s
build (ubuntu-20.04, tool)	1m 0s
build (ubuntu-20.04, cpp)	15m 52s
cpp-lib-build (windows-2022, clang, OFF)	4m 31s
cpp-lib-build (windows-2022, cl)	4m 5s
build (ubuntu-20.04, csharp)	5m 10s
build (ubuntu-20.04, dart)	4m 12s
build (ubuntu-20.04, go)	1m 11s
build (ubuntu-20.04, java)	1m 7s
build (ubuntu-20.04, javascript)	1m 24s
build (ubuntu-20.04, typescript)	9m 53s
build (ubuntu-20.04, php)	1m 24s
build (ubuntu-20.04, python3)	1m 3s
build (windows-2022, tool)	1m 34s
build (windows-2022, cpp)	22m 2s
build (windows-2022, csharp)	7m 38s
build (windows-2022, dart)	4m 26s
build (windows-2022, go)	3m 20s
build (windows-2022, java)	1m 56s
build (windows-2022, javascript)	3m 17s
build (windows-2022, typescript)	19m 25s
build (windows-2022, php)	3m 11s
build (windows-2022, python3)	1m 48s
	3h 10m 9s

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