Update libmpdec-related instructions

* drop --with-system-libmpdec on macOS (default to yes since 3.13)
* recommend deb.sury.org for Debian 12 and Ubuntu 24.04

Author: skirpichev

getting-started/setup-building.rst

@@ -783,6 +783,7 @@ some of CPython's modules (for example, ``zlib``).
 
    Note that Debian 12 and Ubuntu 24.04 do not have the ``libmpdec-dev`` package.  You can safely
    remove it from the install list above and the Python build will use a bundled version.
+   Alternatively, you can install this package from https://deb.sury.org/.
 
 .. tab:: macOS
 
@@ -823,7 +824,6 @@ some of CPython's modules (for example, ``zlib``).
             $ GDBM_CFLAGS="-I$(brew --prefix gdbm)/include" \
                GDBM_LIBS="-L$(brew --prefix gdbm)/lib -lgdbm" \
                ./configure --with-pydebug \
-                           --with-system-libmpdec \
                            --with-openssl="$(brew --prefix openssl@3)"
 
       .. tab:: Python 3.11-3.12