Remove deprecated benchmark::internal::Benchmark usage

[bdice]

This PR fixes build warnings like:

[116/229] sccache $BUILD_PREFIX/bin/x86_64-conda-linux-gnu-c++ -DBENCHMARK_STATIC_DEFINE -DCCCL_DISABLE_PDL -DCUB_DISABLE_NAMESPACE_MAGIC -DCUB_IGNORE_NAMESPACE_MAGIC_ERROR -DRAPIDSMPF_HAVE_CUPTI -DRAPIDSMPF_HAVE_MPI -DRAPIDSMPF_HAVE_NUMA -DRAPIDSMPF_HAVE_SLURM -DRAPIDSMPF_HAVE_STREAMING -DRAPIDSMPF_HAVE_UCXX -DTHRUST_DEVICE_SYSTEM=THRUST_DEVICE_SYSTEM_CUDA -DTHRUST_DISABLE_ABI_NAMESPACE -DTHRUST_HOST_SYSTEM=THRUST_HOST_SYSTEM_CPP -DTHRUST_IGNORE_ABI_NAMESPACE_ERROR -DUCXX_ENABLE_RMM -I$SRC_DIR/cpp/include -I$PREFIX/include/rapids -I$SRC_DIR/cpp/build/_deps/benchmark-src/include -isystem $SRC_DIR/cpp/build/_deps/libcoro-src/include -isystem $SRC_DIR/cpp/build/_deps/libcoro-build/include -isystem $BUILD_PREFIX/include -fvisibility-inlines-hidden -fmessage-length=0 -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -pipe -isystem $PREFIX/include   -I$PREFIX/targets/x86_64-linux/include -I$PREFIX/targets/x86_64-linux/include/cccl -I$BUILD_PREFIX/targets/x86_64-linux/include -I$BUILD_PREFIX/targets/x86_64-linux/include/cccl -O3 -DNDEBUG -std=gnu++20 -fPIE -Wall -Werror -Wextra -Wsign-conversion -Wno-unknown-pragmas -Wno-missing-field-initializers -Wno-error=deprecated-declarations -fexceptions -pipe "-ffile-prefix-map=$PREFIX/=''" -MD -MT benchmarks/CMakeFiles/bench_memory_resources.dir/bench_memory_resources.cpp.o -MF benchmarks/CMakeFiles/bench_memory_resources.dir/bench_memory_resources.cpp.o.d -o benchmarks/CMakeFiles/bench_memory_resources.dir/bench_memory_resources.cpp.o -c $SRC_DIR/cpp/benchmarks/bench_memory_resources.cpp
$SRC_DIR/cpp/benchmarks/bench_memory_resources.cpp:320:55: warning: 'benchmark::internal::Benchmark' is deprecated: Use ::benchmark::Benchmark instead [-Wdeprecated-declarations]
  320 | void CustomArguments(benchmark::internal::Benchmark* b) {
      |                                                       ^
In file included from $SRC_DIR/cpp/benchmarks/bench_memory_resources.cpp:8:
$SRC_DIR/cpp/build/_deps/benchmark-src/include/benchmark/benchmark.h:1386:28: note: declared here
 1386 |     ::benchmark::Benchmark Benchmark;
      |                            ^~~~~~~~~
$SRC_DIR/cpp/benchmarks/bench_memory_resources.cpp:404:77: warning: 'benchmark::internal::Benchmark' is deprecated: Use ::benchmark::Benchmark instead [-Wdeprecated-declarations]
  404 | void PinnedFirstAlloc_InitialPoolSize_Args(benchmark::internal::Benchmark* b) {
      |                                                                             ^
$SRC_DIR/cpp/build/_deps/benchmark-src/include/benchmark/benchmark.h:1386:28: note: declared here
 1386 |     ::benchmark::Benchmark Benchmark;
      |                            ^~~~~~~~~

[nirandaperera]

LGTM

[nirandaperera]

/merge