[CI] Switch to LLVM 21 (#618) #3956

Job	Run time
pre-commit / pre-commit	1m 25s
mac / clang-build (Release)	2m 1s
mac / clang-build (Debug)	1m 58s
ubuntu / clang-build (ubuntu-24.04-arm)	1m 55s
ubuntu / clang-build (ubuntu-24.04)	2m 51s
ubuntu / gcc-build (ubuntu-24.04, Release)	1m 33s
ubuntu / gcc-build (ubuntu-24.04, Debug)	1m 19s
ubuntu / gcc-build (ubuntu-24.04-arm, Debug)	1m 15s
ubuntu / gcc-build (ubuntu-24.04-arm, Release)	1m 40s
windows / msvc-build (Release)	2m 40s
windows / msvc-build (Debug)	2m 43s
windows / clang-build	2m 36s
ubuntu / gcc-test (ubuntu-24.04)	8m 25s
ubuntu / gcc-test (ubuntu-24.04-arm)	8m 15s
mac / clang-test	4m 49s
windows / clang-test	3m 5s
windows / msvc-test	4m 42s
ubuntu / clang-test (ubuntu-24.04)	8m 19s
ubuntu / clang-test (ubuntu-24.04-arm)	8m 23s
ubuntu / clang-sanitizer-build (ubuntu-24.04)	3m 0s
windows / clang-test-extended	2m 55s
ubuntu / clang-sanitizer-test (ubuntu-24.04)	2m 56s
mac / clang-test-extended	2m 57s
windows / msvc-test-extended	2m 44s
ubuntu / clang-sanitizer-test-extended (ubuntu-24.04)	2m 26s
ubuntu / gcc-test-extended (ubuntu-24.04)	4m 29s
ubuntu / gcc-test-extended (ubuntu-24.04-arm)	4m 33s
ubuntu / clang-test-extended (ubuntu-24.04)	4m 31s
ubuntu / clang-test-extended (ubuntu-24.04-arm)	4m 29s
ubuntu / gcc-build-codecov	6m 57s
perf / macos-clang-build-perf-stats	1m 54s
perf / ubuntu-gcc-build-perf-stats	1m 38s
pages / build-scoreboard	17s
pages / build-doxygen-xml	57s
pages / build-sphinx	1m 38s
pages / deploy-pages	12s
	1h 58m 27s

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