sim: Use the [[deprecated()]] attribute instead of GEM5_DEPRECATED.
sim: Use the [[deprecated()]] attribute instead of GEM5_DEPRECATED.

The [[deprecated()]] attribute is now standard c++, so use that
directly.

Change-Id: I246551b05484a707ac9da05dc91af34d78aae1ff
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48507
Reviewed-by: Daniel Carvalho <91779fc734d358d9575ec4910de29bfcbcaf1b22@yahoo.com.br>
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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