[libc++][docs][NFC] Update supported OpenXL version (#127072)
According to the documentation:
The base LLVM version was changed between minor versions:
- 17.1.2 is based on LLVM17
- 17.1.3 is based on LLVM19
https://www.ibm.com/docs/en/openxl-c-and-cpp-aix/17.1.2?topic=whats-new
https://www.ibm.com/docs/en/openxl-c-and-cpp-aix/17.1.3?topic=whats-new
Co-authored-by: Hristo Hristov <zingam@outlook.com>