[web/request] Restore attribute setting for search_state
This was dropped in 1f84295bfe95 when `search_state` attribute was turned into
a property, but without a setter. So adding a setter here restoring previous
behaviour, as some tests and cubes use it.
Related to #4875761.
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/