[libc++] Run back-deployment CI on macOS 10.15 instead of 10.14

The goal was to add coverage for back-deployment over the filesystem
library, but it was added in macOS 10.15, not 10.14.

Differential Revision: https://reviews.llvm.org/D92937

GitOrigin-RevId: 717b0da7a6ef0a8a03d7296324f76132118fcf54
1 file changed