docs/manual: update host gcc minimum required version

The oldest gcc that is known to work with current Buildroot is that of
RHEL 6, version 4.4.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

docs/manual/prerequisite.txt

@@ -23,8 +23,8 @@ between distributions).
 ** +make+ (version 3.81 or any later)
 ** +binutils+
 ** +build-essential+ (only for Debian based systems)
-** +gcc+ (version 2.95 or any later)
-** `g++` (version 2.95 or any later)
+** +gcc+ (version 4.4 or any later)
+** `g++` (version 4.4 or any later)
 ** +bash+
 ** +patch+
 ** +gzip+