Update Mac installer messages to 3.12 alpha (GH-98575)

Mac/BuildScript/resources/ReadMe.rtf

-{\rtf1\ansi\ansicpg1252\cocoartf2580
+{\rtf1\ansi\ansicpg1252\cocoartf2639
 \cocoatextscaling0\cocoaplatform0{\fonttbl\f0\fswiss\fcharset0 Helvetica;\f1\fswiss\fcharset0 Helvetica-Bold;\f2\fswiss\fcharset0 Helvetica-Oblique;
 \f3\fmodern\fcharset0 CourierNewPSMT;\f4\fmodern\fcharset0 Courier;}
 {\colortbl;\red255\green255\blue255;}
@@ -11,7 +11,7 @@
 \pard\tx720\tx1440\tx2160\tx2880\tx3600\tx4320\tx5040\tx5760\tx6480\tx7200\tx7920\tx8640\partightenfactor0
 
 \f1\b \cf0 NOTE: 
-\f0\b0 This is an alpha preview of Python 3.11.0, the next feature release of Python 3.  It is not intended for production use.\
+\f0\b0 This is an alpha preview of Python 3.12.0, the next feature release of Python 3.  It is not intended for production use.\
 \pard\tx720\tx1440\tx2160\tx2880\tx3600\tx4320\tx5040\tx5760\tx6480\tx7200\tx7920\tx8640\pardirnatural\partightenfactor0
 \cf0 \
 \pard\tx720\tx1440\tx2160\tx2880\tx3600\tx4320\tx5040\tx5760\tx6480\tx7200\tx7920\tx8640\pardirnatural\partightenfactor0

Mac/BuildScript/resources/Welcome.rtf

-{\rtf1\ansi\ansicpg1252\cocoartf2580
+{\rtf1\ansi\ansicpg1252\cocoartf2639
 \cocoascreenfonts1\cocoatextscaling0\cocoaplatform0{\fonttbl\f0\fswiss\fcharset0 Helvetica;\f1\fswiss\fcharset0 Helvetica-Bold;\f2\fmodern\fcharset0 CourierNewPSMT;
 }
 {\colortbl;\red255\green255\blue255;}
@@ -26,5 +26,5 @@ At the end of this install, click on
 \
 
 \f1\b NOTE: 
-\f0\b0 This is an alpha test preview of Python 3.11.0, the next feature release of Python 3.  It is not intended for production use.\
+\f0\b0 This is an alpha test preview of Python 3.12.0, the next feature release of Python 3.  It is not intended for production use.\
 }
\ No newline at end of file