Werk #13473: Do not drop services of incompatibly changed plugins

In rare cases a cmk-update-config could silently drop the service of a plugin (see below for which plugins can be affected). It would then appear as stale in the service view. A rediscovery would fix the issue (and usually report one or more new services).

Now the affected service will go to UNKNOWN, and include <missing an item> in the service description. However the required action remains the same: Perform a rediscovery. It will now report the affected service as being vanished (and usually one or more new services).

This only affects services of (custom) plugins, whose implementation was changed: From being a single service plugin ("without item") to being a multiple services plugin ("with item"). In the case a shipped plugin is changed in this regard, this scenario should be avoided by implementing an automatic transition. For custom plugins however, you may encounter the described issue.

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