[Chapel Merge] Use clang/llvm15 for centos 9

Branch: refs/heads/main
Revision: b00d5a82edbd13d622f5d9a03a0d676c67b69dec
Author: jabraham17
Link: Use clang/llvm15 for centos 9 by jabraham17 · Pull Request #28161 · chapel-lang/chapel · GitHub
Log Message:
Use clang/llvm15 for centos 9 (#28161)

Use clang/llvm 15 for centos 9, since apparently clang16 doesn't exist
in the EPEL for centos 9 (but clang16-devel does)

[Reviewed by @benharsh]

Compare: Comparing 256de2da3211f98e275d883307c29983740f8a3a...8d5eeed17c1ac9894d686cc08f686fb0e0883285 · chapel-lang/chapel · GitHub

Diff:
M doc/rst/usingchapel/prereqs-commands.rst
M util/devel/test/portability/apptainer/current/centos-stream-9/image.def
A util/devel/test/portability/provision-scripts/dnf-llvm15.sh
D util/devel/test/portability/provision-scripts/dnf-llvm16.sh
https://github.com/chapel-lang/chapel/pull/28161.diff