net-mgmt/icinga2: Update to 2.13.6

Changelog:
The main focus of version 2.13.6 is improved performance of Icinga DB
and apply rules. Additionally, it includes bug fixes related to config
loading and API permissions.
Bugfixes:
-Improve the throughput of the Icinga DB feature. #9550
-Multiple changes to speed up evaluation of apply rules. #9559 #9565 #9558
-Fix a possible crash on config loading related to ignore_on_error.#9560
-Check API user permission on objects returned by joins. #9561
-Windows: update bundled Boost and OpenSSL versions. #9562 #95

Sponsored by: Netzkommune GmbH