Describe the enhancement requested

I'm seeing some PRs start to prefer the nested namespace notation. I know clang-tidy prefers it. It is more compact and now that we are on C++ 17 I don't see any reason we shouldn't move everything over.

Before:

namespace arrow {
namespace util {
namespace internal {

After:

namespace arrow::util::internal {

Sub issues:

• [C++][Parquet] Use C++17's nested namespaces #36643
• ...

Component(s)

C++