[CI] Switch to LLVM 21 (#618) #3955

Job	Run time
pre-commit / pre-commit	1m 22s
ubuntu / clang-build (ubuntu-24.04)	57s
ubuntu / clang-build (ubuntu-24.04-arm)	1m 53s
ubuntu / gcc-build (ubuntu-24.04, Debug)	50s
ubuntu / gcc-build (ubuntu-24.04, Release)	1m 19s
ubuntu / gcc-build (ubuntu-24.04-arm, Debug)	1m 17s
mac / clang-build (Debug)	1m 57s
ubuntu / gcc-build (ubuntu-24.04-arm, Release)	1m 32s
windows / msvc-build (Debug)	2m 44s
windows / msvc-build (Release)	2m 51s
windows / clang-build	1m 56s
mac / clang-build (Release)	1m 38s
ubuntu / gcc-test (ubuntu-24.04)	8m 27s
ubuntu / gcc-test (ubuntu-24.04-arm)	8m 27s
ubuntu / clang-sanitizer-build (ubuntu-24.04)	54s
ubuntu / clang-test (ubuntu-24.04)	8m 22s
ubuntu / clang-test (ubuntu-24.04-arm)	8m 19s
windows / clang-test	2m 44s
mac / clang-test	4m 49s
windows / msvc-test	4m 39s
ubuntu / clang-sanitizer-test (ubuntu-24.04)	2m 54s
windows / clang-test-extended	2m 55s
ubuntu / clang-sanitizer-test-extended (ubuntu-24.04)	2m 22s
mac / clang-test-extended	2m 53s
windows / msvc-test-extended	3m 6s
ubuntu / gcc-test-extended (ubuntu-24.04)	4m 33s
ubuntu / gcc-test-extended (ubuntu-24.04-arm)	4m 38s
ubuntu / clang-test-extended (ubuntu-24.04-arm)	4m 30s
ubuntu / clang-test-extended (ubuntu-24.04)	4m 30s
ubuntu / gcc-build-codecov	5m 58s
perf / ubuntu-gcc-build-perf-stats	1m 36s
perf / macos-clang-build-perf-stats	1m 35s
pages / build-scoreboard	19s
pages / build-doxygen-xml	2m 20s
pages / build-sphinx	1m 20s
pages / deploy-pages	0s
	1h 52m 26s

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