Werk #16689: Decommission legacy check API

This werk only affects users and maintainers of custom check plugins developed against the API that was replaced in Checkmk 2.0.0.

The old API for the plugins residing in local/share/check_mk/checks will no longer be stable in Checkmk version 2.3. Plugins not maintained by Checkmk will almost certainly be incompatible and therefore ignored.

Compatible plugins will be considered, but the notion of what constitutes a compatible plugin may change any time.

As of Checkmk version 2.2 plugins in that folder generated a warning on the commandline and resulted in an "Analyze Configuration" test issueing a WARNING.

We now escalate this to CRITICAL. Incompatible plugins are reported during omd update.

If you maintian such plugins, please migrate them to the new API before upgrading to Checkmk 2.3. You can find a blog post on how to migrate these plugins here. A comprehensive guide on how to write plugins is found in our documentation. Please also refer to the APIs documentation found in your site (Help -> Check plugin API reference).

Commandline call plugins for special agents and active checks in this folder will still work, but we provide a new API for those as well now (see Werk #16259). They will stop working in Checkmk 2.4.

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