Cirrus-CI: use llvm16 job by default and make llvm15 manual

With commit e048f78b2143 (head) / 2e7b90a28f69 (stable/13) the default
tool chain is Clang/LLVM 16, so follow along in the llvm pkg used by
Cirrus-CI.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 171f1c1b4f0e9ec5b77a2dabdd4fcbf9baaa02df)