CORS issue login staging

[f1sh1918]

Describe the bug
As a tester i want to login on staging for testing. When i try to login i get "Server nicht erreichbar"

To Reproduce
Steps to reproduce the behavior:

1. [Go to '...'](https://staging.bayern.ehrenamtskarte.app/)
2. Try to login (without a CORS plugin)
3. Check error message

Access to fetch at 'https://api.staging.entitlementcard.app/' from origin 'https://staging.bayern.ehrenamtskarte.app' has been blocked by CORS policy: The 'Access-Control-Allow-Origin' header contains multiple values '*, https://staging.bayern.ehrenamtskarte.app', but only one is allowed. Have the server send the header with a valid value, or, if an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

Expected behavior
Login works without cors issue

Additional context
We changed the backend config flag production: false for staging environment, since we wanted to expose an endpoint only for staging. This is causing the issue. We may introduce a different flag for securing the endpoint or adjusting the CORS configuration here:
8201c96

[f1sh1918]

i will change the production value in salt after this was released. That should fix the isse
#1417

[f1sh1918]

fixed by this pr
https://git.tuerantuer.org/DF/salt/pulls/255/files