[CI] Switch to LLVM 21 #3953

Job	Run time
pre-commit / pre-commit	1m 11s
ubuntu / clang-build (ubuntu-24.04)	1m 49s
ubuntu / gcc-build (ubuntu-24.04-arm, Debug)	1m 38s
ubuntu / gcc-build (ubuntu-24.04-arm, Release)	1m 35s
ubuntu / gcc-build (ubuntu-24.04, Release)	1m 27s
ubuntu / gcc-build (ubuntu-24.04, Debug)	1m 22s
mac / clang-build (Debug)	1m 57s
mac / clang-build (Release)	2m 42s
ubuntu / clang-build (ubuntu-24.04-arm)	1m 44s
windows / msvc-build (Debug)	2m 49s
windows / msvc-build (Release)	3m 12s
windows / clang-build	2m 27s
ubuntu / gcc-test (ubuntu-24.04)	8m 23s
ubuntu / gcc-test (ubuntu-24.04-arm)	8m 24s
ubuntu / clang-sanitizer-build (ubuntu-24.04)	2m 48s
ubuntu / clang-test (ubuntu-24.04)	8m 36s
ubuntu / clang-test (ubuntu-24.04-arm)	8m 21s
windows / clang-test	3m 14s
mac / clang-test	4m 51s
windows / msvc-test	4m 55s
ubuntu / clang-sanitizer-test (ubuntu-24.04)	2m 55s
windows / clang-test-extended	3m 24s
mac / clang-test-extended	3m 8s
ubuntu / clang-sanitizer-test-extended (ubuntu-24.04)	2m 31s
windows / msvc-test-extended	3m 21s
ubuntu / gcc-test-extended (ubuntu-24.04)	4m 27s
ubuntu / gcc-test-extended (ubuntu-24.04-arm)	4m 42s
ubuntu / clang-test-extended (ubuntu-24.04-arm)	4m 29s
ubuntu / clang-test-extended (ubuntu-24.04)	4m 44s
ubuntu / gcc-build-codecov	6m 32s
perf / ubuntu-gcc-build-perf-stats	1m 33s
perf / macos-clang-build-perf-stats	1m 46s
pages / build-scoreboard	16s
pages / build-doxygen-xml	1m 11s
pages / build-sphinx	1m 29s
pages / deploy-pages	0s
	1h 59m 53s

Provide feedback