Bump our macOS target version to 10.9.

https://developer.apple.com/documentation/xcode/notarizing_macos_software_before_distribution/resolving_common_notarization_issues
says "notarization only works for binaries linked against macOS 10.9
or later".

Test: N/A
Bug: http://b/143105198
Change-Id: If66f81caa2f074bf70a75094da578295aa9f58ad