apacheGH-45053: [C++] Add support for Boost 1.87.0 (apache#45057)

### Rationale for this change

Boost 1.87.0 removed `BOOST_PROCESS_V2_ASIO_NAMESPACE`:
boostorg/process@e827d14

### What changes are included in this PR?

Use `BOOST_PROCESS_V2_NAMESPACE::net` instead.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: apache#45053

Authored-by: Sutou Kouhei <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: kou

cpp/src/arrow/testing/process.cc

@@ -85,9 +85,14 @@
 #  include <thread>
 
 #  ifdef BOOST_PROCESS_USE_V2
-namespace asio = BOOST_PROCESS_V2_ASIO_NAMESPACE;
 namespace process = BOOST_PROCESS_V2_NAMESPACE;
 namespace filesystem = process::filesystem;
+// For Boost < 1.87.0
+#    ifdef BOOST_PROCESS_V2_ASIO_NAMESPACE
+namespace asio = BOOST_PROCESS_V2_ASIO_NAMESPACE;
+#    else
+namespace asio = process::net;
+#    endif
 #  elif defined(BOOST_PROCESS_HAVE_V1)
 namespace process = boost::process::v1;
 namespace filesystem = boost::process::v1::filesystem;