text: Title: Deprecate `accept_plain_values` argument used by `timestr_to_secs`
  Body: This argument was added in 579667dfc0dcdeaa62863dc4ce690d06739ae521 when adding limit to WHILE loops. WHILE loop limits were changed in 628d405eb622918778791997efadcbe23cf7d486 so that the argument isn't anymore used for anything by Robot itself. It's not that useful in the public API and it's behavior hasn't been documented. Best to deprecate it now and remove later.