Upgrade to go 1.21, replace golang.org/x/exp/slices with slices #1134

Job	Run time
cpp-lib-build (macos-12, clang, ON)	1m 2s
cpp-lib-build (macos-12, clang, OFF)	1m 0s
build (macos-12, tool)	1m 55s
cpp-lib-build (macos-12, gcc, ON)	1m 10s
build (macos-12, cpp)	20m 55s
cpp-lib-build (macos-12, gcc, OFF)	1m 13s
build (macos-12, csharp)	6m 47s
cpp-lib-build (ubuntu-20.04, clang, ON)	1m 5s
build (macos-12, dart)	8m 1s
cpp-lib-build (ubuntu-20.04, clang, OFF)	57s
build (macos-12, go)	2m 35s
cpp-lib-build (ubuntu-20.04, gcc, ON)	2m 11s
build (macos-12, java)	2m 29s
cpp-lib-build (ubuntu-20.04, gcc, OFF)	2m 25s
build (macos-12, javascript)	2m 11s
cpp-lib-build (windows-2022, clang, ON)	4m 10s
build (macos-12, typescript)	15m 25s
cpp-lib-build (windows-2022, clang, OFF)	4m 51s
build (macos-12, php)	3m 31s
cpp-lib-build (windows-2022, cl)	4m 22s
build (macos-12, python3)	2m 9s
build (ubuntu-20.04, tool)	1m 0s
build (ubuntu-20.04, cpp)	15m 40s
build (ubuntu-20.04, csharp)	5m 5s
build (ubuntu-20.04, dart)	3m 58s
build (ubuntu-20.04, go)	1m 21s
build (ubuntu-20.04, java)	1m 12s
build (ubuntu-20.04, javascript)	1m 23s
build (ubuntu-20.04, typescript)	10m 13s
build (ubuntu-20.04, php)	1m 12s
build (ubuntu-20.04, python3)	1m 5s
build (windows-2022, tool)	1m 27s
build (windows-2022, cpp)	23m 19s
build (windows-2022, csharp)	7m 32s
build (windows-2022, dart)	4m 51s
build (windows-2022, go)	4m 43s
build (windows-2022, java)	1m 46s
build (windows-2022, javascript)	3m 0s
build (windows-2022, typescript)	19m 31s
build (windows-2022, php)	2m 12s
build (windows-2022, python3)	2m 59s
	3h 23m 53s

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