mem-garnet: Upgrade garnet version to 3.0

mem-garnet: Upgrade garnet version to 3.0

This version of garnet includes HeteroGarnet which
supports heterogenous interconnect systems, flexible
router and link configurations, and better debugging
resources.
This patch changes the garnet directory structure
to not include the version number. The user will be
informed about the garnet version being used.

Change-Id: Id4763421528305193ae0cd10c159b385a9513553
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34259
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>