Fixed agent limit logic (#4560)

* Fixed agent limit logic

As mentioned in #4199 the logic for verifying if an additional agent would exceed the maximum of agents is off by one.
This commit fixes that.

* Revert "Fixed agent limit logic"

This reverts commit f7cde3cb019bd1f4cb16805f5c8c2234c2f138af.

* Fix maximum agent limit in the shared agent key library.

* Update changelog

* Update changelog

Co-authored-by: Vikman Fernandez-Castro <victor@wazuh.com>