scons: Set the minimum scons version to 3.0.
scons: Set the minimum scons version to 3.0.

Change-Id: Id57a93e819588d2231d2d2d8b28cd62b05fbbe9b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33675
Reviewed-by: Steve Reinhardt <3d08042c05fa8d209ba80a9a1b89252630788148@gmail.com>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5