upgrade to .net8 (#3)

.net6 upgrade (#2)

* .net6 upgrade

* add gh action

* checkout submodules

* remove appveyor

* Update action versions

The used versions of the github actions are now eol, so updating to more recent
ones.

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Co-authored-by: Bernhard Dick <bernhard@bdick.de>

additional error handling to prevent core dumps

handle connection errors

fix rfp id fallback

update framework

update build

use RFP name in graphite path

update mitel-api

handle empty response (rfcpcount % batchsize == 0)