Revert "[infra] Use Mac10.15.7 and xcode12.3 for all builds."

This reverts commit f5aed172c69300b3314f9558125fa3d303950893.

Reason for revert: Broke 11phone11 (ios 13.6) testing.

Original change's description:
> [infra] Use Mac10.15.7 and xcode12.3 for all builds.
>
> Bug: skia:11129
> Change-Id: I314d51988e4cafc9dfea60bc70a57f46faa8666a
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/345396
> Reviewed-by: Eric Boren <borenet@google.com>

TBR=borenet@google.com,westont@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: skia:11129
Change-Id: Ib4484042ebaf493edcc8c5ee21960251bc80aa42
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/352496
Reviewed-by: Weston Tracey <westont@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
11 files changed