Update xcode up to 12.4.0

[merkispavel]

Issue

Circle is deprecating xcode 11 so we need to upgrade(see https://circleci.com/docs/2.0/testing-ios/)

tldr: xcode 12.4.0 works out of the box without any other fixes, the newest xcode doesn't

What about newest xcode version(13.12.1)

I tried different xcode versions. It looks like everything works smoothly on mac os 10.* while we need fixes for mac os 11(i.e xcode version >= 12.5.1)

What fixes do we need for the newest xcode?

1. The first issue was six python package: six is installed by default and there's a conflict when you try to install protobuf via brew(six is a dependency of protobuf).

• the fix is to remove default "six": pip3 uninstall -y six(see e3fb8e9)

2. After that another issue popped up: prime server wasn't able to build successfully(see build logs)

m4/ax_pthread.m4:88: AX_PTHREAD is expanded from...
configure.ac:37: the top level
configure.ac:2: error: possibly undefined macro: m4_esyscmd
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.
autoreconf: error: /usr/local/Cellar/autoconf/2.71/bin/autoconf failed with exit status: 1

At this moment I gave up and tried to decrease xcode version one more time. And it worked

fixes #3490