CI: force macOS Kernel version to 15

It corresponds to macOS 10.11 El Capitan ¹ which is the oldest macOS version VLC 4 is currently supposed to support ².

I don't know if it's supposed to tell the minimum set of supported API. I suppose so as the value is used in the triplet. On 3.0 the values are 18 and 19 in the CI, which doesn't seem to match the desired targets either. But they are not used to make the release binaries (nor the 4.0 CI).

1. https://en.wikipedia.org/wiki/MacOS_version_history#Overview ↩

2. https://code.videolan.org/videolan/vlc/-/blob/dd8071a150d28c7abcdea6429c7a452a25150b3f/NEWS#L7 ↩