Warn against toilet=* instead of toilets=*

[Famlam]

Per the wiki page of ​amenity=toilets:

Please do not use toilet=yes (singular).

There are about ​5k instances of toilet, of which 4k ways in combination with building=* and 1k nodes.

Perhaps it is possible to add the following rule to JOSM:

*[toilet] {
  throwWarning: tr("{0} is deprecated", "{0.key}");
  suggestAlternative: "toilets";
  group: tr("deprecated tagging");
}

Originally filed at ​https://github.com/osm-fr/osmose-backend/issues/1447

[Klumbumbus]

In 18545/josm:

Fix #22335 - Deprecate toilet in favor of toilets