Work on check-newest-clang: Switch from clang 20 to clang 21. #124

Job	Run time
build-tarball	3m 29s
make check on Ubuntu GNU/Linux	0s
make check on Solaris 11	0s
make check on FreeBSD	0s
make check on OpenBSD	0s
make check on CentOS GNU/Linux	0s
make check on macOS	0s
make check on Cygwin	0s
make check on AlmaLinux GNU/Linux	0s
make check on NetBSD	0s
make check on Solaris 11 OmniOS	0s
make check with the newest clang release	0s
make check on mingw	0s
make check with a cross-compiler	0s
make check on Alpine Linux	0s
make check with sanitizers	0s
make check on MSVC	0s
	3m 29s

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