[CI] Switch to LLVM 21 #3952

Job	Run time
pre-commit / pre-commit	1m 6s
ubuntu / gcc-build (ubuntu-24.04, Release)	55s
ubuntu / gcc-build (ubuntu-24.04, Debug)	56s
ubuntu / gcc-build (ubuntu-24.04-arm, Debug)	1m 23s
windows / msvc-build (Debug)	3m 15s
ubuntu / gcc-build (ubuntu-24.04-arm, Release)	50s
windows / clang-build	2m 20s
mac / clang-build (Release)	2m 5s
mac / clang-build (Debug)	1m 46s
windows / msvc-build (Release)	2m 9s
ubuntu / clang-build (ubuntu-24.04)	1m 0s
ubuntu / clang-build (ubuntu-24.04-arm)	1m 14s
ubuntu / clang-test	0s
ubuntu / clang-sanitizer-build	0s
ubuntu / clang-sanitizer-test	0s
ubuntu / clang-test-extended	0s
ubuntu / clang-sanitizer-test-extended	0s
ubuntu / gcc-test (ubuntu-24.04-arm)	8m 19s
ubuntu / gcc-test (ubuntu-24.04)	8m 21s
windows / clang-test	3m 0s
mac / clang-test	4m 53s
windows / msvc-test	4m 52s
windows / clang-test-extended	3m 2s
mac / clang-test-extended	3m 11s
windows / msvc-test-extended	2m 59s
ubuntu / gcc-test-extended (ubuntu-24.04-arm)	4m 33s
ubuntu / gcc-test-extended (ubuntu-24.04)	4m 46s
ubuntu / gcc-build-codecov	-1s
perf	0s
pages	0s
	1h 6m 54s

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