Strip 'iss' query param from the redirect URL for Keycloak auth flow

[mayorova]

What this PR does / why we need it:

Since version 23 Keycloak (RHBK) adds a iss query parameter to the authentication response, see https://www.keycloak.org/docs/latest/upgrading/index.html#migrating-to-23-0-0

The result of this is that 3scale client included this iss value to the redirect_uri when fetching the access token, which in its turn caused an error:

{"error":"invalid_grant","error_description":"Incorrect redirect_uri"}

This PR only removes the iss from the redirect URI when forming the access token request. Ideally, we would need to also verify the value of the iss parameter to make sure it is exactly the same as the Realm URL specified in the SSO configuration. See 2.4. Validating the Issuer Identifier in RFC 9207.

Which issue(s) this PR fixes

https://issues.redhat.com/browse/THREESCALE-11200

Verification steps

Set up a client for dev portal authentication in RHBK v24 and configure dev portal SSO.
Test the integration and verify it's successful even when the compatibility mode "Exclude Issuer From Authentication Response" is OFF.

Special notes for your reviewer:

[jlledom]

The code looks correct and the tests pass.

I tried to launch a Keycloack 24 instance from openshift but it failed. I assume you tried it and it works.