Make safepower BMC agent build with Boost 1.92.

Two Boost APIs used by `//production/borg_mgmt/node_proxy/safepower/safepower_agent/bmc`
changed in newer Boost, so the package does not compile with Boost 1.92
(cl/988960422):

*   `remote_state_ping.cc`: since Boost 1.86, `<boost/process.hpp>` is
    Boost.Process v2, and v1 lives in `boost/process/v1/` (namespace
    `boost::process::v1`). When `boost/process/v1.hpp` exists, include it and
    use `boost::process::v1`. Otherwise (Boost 1.83) keep using
    `<boost/process.hpp>` and `boost::process`. The code uses a local `bp`
    namespace alias for both cases.
*   `scheduler_bmc.cc`: `io_context::poll(error_code&)` was removed in Boost
    1.87. Call `io_context::poll()`, which exists in both versions. The
    `error_code` overload never reported an error, so no check is lost.

PiperOrigin-RevId: 991040099
Change-Id: I7e6bcc5fcd3acf0e23e7b0aeea88b081ddbd33f2