Merge pull request #9442 from MikeMcQuaid/catalina-ruby-fix

brew.sh: don't allow system Ruby on Catalina.

Library/Homebrew/brew.sh

@@ -341,7 +341,9 @@ then
 
   # Set a variable when the macOS system Ruby is new enough to avoid spawning
   # a Ruby process unnecessarily.
-  if [[ "$HOMEBREW_MACOS_VERSION_NUMERIC" -lt "101500" ]]
+  # On Catalina the system Ruby is technically new enough but don't allow it:
+  # https://github.com/Homebrew/brew/issues/9410
+  if [[ "$HOMEBREW_MACOS_VERSION_NUMERIC" -lt "101600" ]]
   then
     unset HOMEBREW_MACOS_SYSTEM_RUBY_NEW_ENOUGH
   else