Werk #14961: MRPE: Deprecate add_age flag

Previously, there was an option to decide whether the cache age (in case of activated caching) of an MRPE check should be displayed at the corresponding service. This could be specified either at the "Execute MRPE checks" agent ruleset, or directly at the mrpe.cfg file (UNIX) or check_mk.user.yml file.

This option turned out to be of limited use, as there's no advantage in not including the cache age information to the service. Hence, the new behavior is to always include the cache age information.

While this option has been inactive for a while (and never has been active for Windows), it's now officially removed from the config format. Old agent rulesets will be migrated automatically; manually specified checks will continue to work, but the appendage/add_age flags will be ignored without further notice.

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