Update macOS version for build workflow

[ryfactor]

Update to macos-15-intel. Note this is the last intel version before ARM64. See actions/runner-images#13046 for further details.

[ryfactor]

I get the following error on a macos-15-intel runner:

[ 32%] Building CXX object audio/CMakeFiles/FeAudio.dir/RtMidi.cpp.o
/Users/runner/work/ivan/ivan/audio/RtMidi.cpp:1035:15: error: variable length arrays in C++ are a Clang extension [-Werror,-Wvla-cxx-extension]
 1035 |   Byte buffer[nBytes+(sizeof(MIDIPacketList))];
      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/runner/work/ivan/ivan/audio/RtMidi.cpp:1035:15: note: read of non-const variable 'nBytes' is not allowed in a constant expression
/Users/runner/work/ivan/ivan/audio/RtMidi.cpp:1018:16: note: declared here
 1018 |   unsigned int nBytes = message->size();
      |                ^
1 error generated.
make[2]: *** [audio/CMakeFiles/FeAudio.dir/RtMidi.cpp.o] Error 1
make[1]: *** [audio/CMakeFiles/FeAudio.dir/all] Error 2
make: *** [all] Error 2
Error: Process completed with exit code 2.