Update cpp/benchmarks codeowners to cuopt-solver-codeowners (#1742)

Renames the owning team for `cpp/` and `benchmarks/` in `.github/CODEOWNERS` from `@NVIDIA/cuopt-engine-codeowners` to `@NVIDIA/cuopt-solver-codeowners`.

These were the only two references to the old team name in the repo. No other ownership rules change.

## Issue

NA

Authors:
  - Ramakrishna Prabhu (https://github.com/ramakrishnap-nv)

Approvers:
  - James Lamb (https://github.com/jameslamb)

URL: #1742

Author: ramakrishnap-nv

.github/CODEOWNERS

@@ -2,8 +2,8 @@
 *                  @nvidia/cuopt-infra-codeowners
 
 #cpp code owners
-cpp/               @nvidia/cuopt-engine-codeowners
-benchmarks/        @nvidia/cuopt-engine-codeowners
+cpp/               @nvidia/cuopt-solver-codeowners
+benchmarks/        @nvidia/cuopt-solver-codeowners
 
 #python code owners
 python/            @nvidia/cuopt-infra-codeowners