NEWS, man: move description of SR-IOV-R net naming to v255

NEWS, man: move description of SR-IOV-R net naming to v255

https://github.com/systemd/systemd/pull/29582 adds the "v254" name. This also
changes what the default is and what "latest" refers to. Without the name, the
code could be enabled via runtime configuration. Nevertheless, it could be
enabled at compilation time. In other words:
  meson setup build -Ddefault-net-naming-scheme=v254
would work, but
  net.naming-scheme=v254
would fail.

It is possible that people were using the compile-time override, so I think
we should allow "v254" scheme to stay and clearly document that it wasn't the
default.

Unfortunately, unless people manually introduced the compile-time override, we
were never actually testing the new code too. So all the pull request testing
was not useful.