GHA/macos: Sequoia chores, fixes for llvm 18

GHA/macos: Sequoia chores, fixes for llvm 18

- add `macos-15` Sequoia to combinations jobs.

- add `llvm@18` to combinations jobs for `macos-15`.

- fix cmake, autotools configurations for llvm 18.
  It requires `CMAKE_OSX_SYSROOT` for cmake, and `--sysroot` within `CC`
  for both cmake and autotools.
  (Also add `--target` to `CC` for all jobs.)

- stop `brew update` on `macos-12`.
  `macos-12` is Monterey. Homebrew stopped providing binaries for it
  after the latest macOS was out. It means newer versions of packages
  have to be built from source, which is slow and undesired. Try to
  avoid it using this trick. If it doesn't work, `macos-12` jobs will
  have to be dropped.

- update Xcode support matrix.

Ref: https://github.com/actions/runner-images/blob/main/images/macos/macos-15-arm64-Readme.md
Closes #15108