Revert "build: update minimum Xcode version for macOS"

Ref: https://github.com/nodejs/node/pull/29622 Ref: https://github.com/nodejs/node/pull/29493

Node.js build infrastructure is not prepared to release 13.x on anything but Xcode 8 on macOS 10.11.

We have limited macOS resources in MacStadium that we have to share for all test and build across our different release lines and so far have avoided the complexity of managing multiple versions of Xcode on a single build server.

IBM have stepped up, yet again, to fill the gap here (you really should thank IBM for carrying so much of the infra work, and they're not even responsible for triggering changes to our compiler minimums). See https://github.com/nodejs/build/pull/1967, but if 13.0.0 is going out in a couple of days this is going to get done safely on our release infrastructure and we should probably proceed with status quo.

We could probably bump this in the middle of 13.x and get away with it when we have the release infra safely ready. It would be unprecedented to do this, but our -mmacosx-version-min=10.10 and the macOS compiler toolchain in general gives us some pretty good stability guarantees.

@nodejs/build @nodejs/tsc @nodejs/releasers