[2.3.0] Checkmk Werk 16689 created: Decommission legacy check API

[//]: # (werk v2) # Decommission legacy check API key | value ---------- | --- date | 2024-04-17T06:27:51+00:00 version | 2.3.0b6 class | feature edition | cre component | checks level | 1 compatible | no 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 <i>"Analyze Configuration"</i> 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](https://checkmk.com/blog/migrating-check-plug-ins-to-checkmk-2-0). A comprehensive guide on how to write plugins is found [in our documentation](https://docs.checkmk.com/2.3.0-beta/en/devel_check_plugins.h…. 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](https://checkmk.com/werk/16259)). They will stop working in Checkmk 2.4.