Update README to clarify ulimit requirement on macOS

[xtqqczze]

This PR updates the README.md to clarify that raising the file descriptor limit with:

ulimit -n 2048

is only required on macOS versions prior to 14 (Sonoma).

• Older macOS releases default to a soft limit of 256 file descriptors, which can cause dotnet restore to fail during the bootstrap.sh build.
• Starting with macOS 14 (Sonoma), the default soft limit was increased to 2560, so manually setting the limit is no longer necessary.