Make gbmcweb build with Boost 1.92:

*   `subprocess_utils.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_VERSION >= 108600`, include
    `boost/process/v1.hpp` and use `boost::process::v1`. Otherwise keep using
    `<boost/process.hpp>` and `boost::process`.
*   `log_services.cc` and `log_services_eventlog.cc`:
    `boost::beast::http::field::content_transfer_encoding` was removed from
    Beast in newer Boost. Pass the header name `"Content-Transfer-Encoding"`
    as a string instead.

Google-Bug-Id:567853120
PiperOrigin-RevId: 991112692
Change-Id: Ia3afc0fa3940b85c5c01fc4ecfd9d2e87ac15722