Doc: Require GDB 7.5 (released Aug 2012) with Python 2.7 or 3.3+

The formerly required 7.4.1+x does not produce valid stack traces
through its Python interface, which we nowadays want to use.

The version requirement for GDBserver (7.0+) is not affected,
but the general advice is still to use a GDBserver version
matching the version of GDB if possible.

Change-Id: Icb08544c56f4ba8c16ca77938b87be8a680ac563
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>