[ci] Change llvm-12 to llvm-17

[JSUYA]

ubuntu-latest has been changed to 24.04.
clang-12 is no longer in 24.04 repository.

[JSUYA]

note. #26

[swift-kim]

The build instructions in our README could be updated as well.

Please also check the recent changes in the upstream buildroot (I created this repo 3 years ago) to see if the build can be further optimized (or more restrictions applied) for clang-17. Here are the examples.

embedder/build/config/BUILD.gn

Lines 83 to 95 in 45ccc85

	if (clang_version >= 11) {
	cflags += [ "-Wno-psabi" ]
	}
	if (clang_version >= 14) {
	cflags += [
	"-Wno-unused-but-set-parameter",
	"-Wno-unused-but-set-variable",
	"-Wno-non-c-typedef-for-linkage",
	]
	}
	if (clang_version >= 15) {
	cflags += [ "-Wno-unqualified-std-cast-call" ]
	}

[JSUYA]

I updated PR by referencing the commit history here.
https://github.com/flutter/buildroot/blob/master/build/config/compiler/BUILD.gn
If there's anything else I should check, please let me know.