fix: change synapseDomain to matrixDomain

Change synapseDomain to matrixDomain to fix the problem that the Synapse can't start due to a missing certificate.

When the Merge is done the Configuration in the openDesk Deployment has also to be changed: https://gitlab.opencode.de/bmi/opendesk/deployment/opendesk/-/blame/main/helmfile/apps/services/values-certificates.yaml.gotmpl?ref_type=heads#L8