[skip ci] Add note about ext/imap removal in upgrading docs

The imap extension has been moved to PECL. Adding a note about configure
options changes also in the UPGRADING.INTERNALS for easier build scripts
adjustments.

Author: petk

UPGRADING.INTERNALS

@@ -41,10 +41,12 @@ PHP 8.4 INTERNALS UPGRADE NOTES
  a. Abstract
    - SIZEOF_SHORT symbol removed (size of 2 on 32-bit and 64-bit platforms).
    - DBA_CDB_MAKE symbol removed in ext/dba.
+   - The configure option --with-imap has been removed.
    - The configure option --with-mhash emits deprecation warning.
    - The configure option --with-pspell has been removed.
 
  b. Unix build system changes
+   - The configure option --with-imap-ssl has been removed.
    - The configure option --with-zlib-dir has been removed.
    - COOKIE_IO_FUNCTIONS_T symbol has been removed (use cookie_io_functions_t).
    - HAVE_SOCKADDR_UN_SUN_LEN symbol renamed to HAVE_STRUCT_SOCKADDR_UN_SUN_LEN.