[BUG] client state unknown field upload_backoff

[SplinterHead]

boinc-client returned data for the /v1/client/state endpoint that contained an unknown field so couldn't map it to the Model

...
    </gui_urls>
    <sched_priority>-2.302771</sched_priority>
    <project_files_downloaded_time>1687986297.679503</project_files_downloaded_time>
    <upload_backoff>12843.957831</upload_backoff>
    <project_dir>/var/lib/boinc/projects/www.worldcommunitygrid.org</project_dir>
</project>
...
marshmallow.exceptions.ValidationError: {'client_state': {'project': {0: {'upload_backoff': ['Unknown field.']}}}}
127.0.0.1 - - [26/Jul/2023 11:55:58] "GET /v1/client/state?client={GUID} HTTP/1.1" 500 -