feat: use staging TF API on stage (#542)

As it was suggested by @thrix (@mvadkert), we should use staging API of
Testing Farm on our staging instance to test and validate their changes.

Given the fact there are not many users of Packit staging instance apart
from Packit Team and Packit Validation, we should be able to verify the
TF changes before they land on **both** TF and Packit production, which
should in turn increase overall stability of Testing Farm experience
while using Packit.

secrets/packit/stg/packit-service.yaml.j2

@@ -8,7 +8,7 @@ validate_webhooks: true
 
 {{ vault.packit_service | to_nice_yaml }}
 
-testing_farm_api_url: https://api.dev.testing-farm.io/v0.1/
+testing_farm_api_url: https://api.staging.testing-farm.io/v0.1/
 enabled_projects_for_internal_tf:
   - github.com/packit/hello-world
   - github.com/oamg/convert2rhel