Cirrus-CI: bump Clang versions to 16 and 17

Clang/LLVM 17 is currently the in-tree default compiler, so use it as
the default Cirrus-CI toolchain. Clang/LLVM 18 is coming soon and needs
to be added here, but I ran into trouble with llvm18-lite package
availability so will look at that later.

Reviewed by: dim (earlier), Jose Luis Duran
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D44162