Generic OAuth: Invalid redirect url is returned when server.protocol = h2

### What happened? When server.protocol is set to h2, it is not possible to login with OAuth2 Generic provider. This is because Grafana sets redirect_url to h2://..., instead of the expected https. As a result, browser can't follow the link. ### What did you expect to happen? The redirect_uri parameter, which is constructed from Auth URL, should never include h2 protocol, but either http or https. ### Did this work before? No. ### How do we reproduce it? 1. Configure Grafana server with server.protocol = https in grafana.ini. 2. Configure and test functionality of Generic OAuth provider. 3. In grafana.ini, switch server.protocol to h2 and restart the server. 4. Now it is not possible to log-in with OAuth, unless server.protocol is switched back to https. ### Is the bug inside a dashboard panel? No. ### Environment (with versions)? Grafana: 12.1.1 OS: Debian 13 Browser: Firefox, Microsoft Edge ### Grafana platform? A package manager (APT, YUM, BREW, etc.) ### Datasource(s)? _No response_