[build] Remove support for older Xcode versions

The CI currently uses Xcode 13.0 beta 4. It is very likely that if the build starts failing with Xcode 12.x, this will go unnoticed. Let's warn the users to upgrade to a newer version of Xcode.

Author: egorzhdan

utils/build-script

@@ -48,10 +48,6 @@ from swift_build_support.swift_build_support.utils import fatal_error
 # These versions are community sourced. At any given time only the Xcode
 # version used by Swift CI is officially supported. See ci.swift.org
 _SUPPORTED_XCODE_BUILDS = [
-    ("12.3", "12C33"),
-    ("12.4", "12D4e"),
-    ("12.5", "12E262"),
-    ("13.0 beta", "13A5154h"),
     ("13.0 beta 4", "13A5201i"),
     ("13.0", "13A233"),
     ("13.1 RC 1", "13A1030d"),