official_taps: deprecate homebrew/php.

After all these formulae have been deleted we want to warn people on
tapping it.

Library/Homebrew/official_taps.rb

-OFFICIAL_TAPS = %w[
-  php
-].freeze
-
 OFFICIAL_CASK_TAPS = %w[
   cask
   versions
@@ -25,6 +21,7 @@ DEPRECATED_OFFICIAL_TAPS = %w[
   gui
   head-only
   nginx
+  php
   python
   science
   tex