Drop support for LLVM 16 in main

[steven-johnson]

Per policy, Halide 19 will support LLVM 17, 18, 19 (plus top-of-tree which is 20)

[alexreinking]

Woot!

[alexreinking]

Should probably update this, too, but my forthcoming build updates will handle this anyway:

Halide/dependencies/llvm/CMakeLists.txt

Lines 23 to 25 in c7e1b99

	if (LLVM_PACKAGE_VERSION VERSION_LESS 16.0)
	message(FATAL_ERROR "LLVM version must be 16.0 or newer")
	endif ()