mem-ruby: Replace the deprecated Stats namespace
mem-ruby: Replace the deprecated Stats namespace

Using the old namespace generates quite a few warnings during
compilation time.

Change-Id: Ibbb8f54f0bee974443dc5dfcc30e95100b8c24d6
Signed-off-by: Hoa Nguyen <a0aa8449aacb303edcb25d6bbe69da9124f010fc@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/60289
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Maintainer: Bobby Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>

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