Remove clang-18 from compilers CI #628

Job	Run time
cargo clippy	38s
cargo test	42s
make (check, RUSTC='rustc +1.58.0', 1.58.0)	9m 15s
make (test-bundled-gems, --enable-yjit=dev)	24m 14s
make (check, --enable-yjit=dev, --yjit-call-threshold=1 --yjit-verify-ctx --yjit-code-gc, -T --yj...	17m 2s
make (check, --enable-yjit=dev)	13m 4s
make (yjit-bindgen, To fix: use patch in logs, --with-gcc=clang-14 --enable-yjit=dev, /usr/lib/ll...	4m 43s
YJIT Ubuntu result	2s
	1h 9m 40s

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