[ci skip] Update UPGRADING about Autoconf version

Author: petk

UPGRADING.INTERNALS

@@ -188,6 +188,7 @@ PHP 7.3 INTERNALS UPGRADE NOTES
   a. Unix build system changes
     - PHP_PROG_LEX, TSRM_CHECK_GCC_ARG, and LIBZEND_CPLUSPLUS_CHECKS Autoconf
       macros have been removed.
+    - Minimum Autoconf version is 2.68+ for generating the configure script.
 
   b. Windows build system changes
     - ZEND_WIN32_FORCE_INLINE doesn't affect C++ anymore. zend_always_inline is