[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

Author: ldionne

libcxx/utils/ci/buildkite-pipeline.yml

@@ -276,8 +276,8 @@ steps:
         - exit_status: -1  # Agent was lost
           limit: 2
 
-  - label: "Apple back-deployment macosx10.14"
-    command: "libcxx/utils/ci/run-buildbot x86_64-apple-system-backdeployment-10.14"
+  - label: "Apple back-deployment macosx10.15"
+    command: "libcxx/utils/ci/run-buildbot x86_64-apple-system-backdeployment-10.15"
     artifact_paths:
       - "**/test-results.xml"
     agents: