Support Keycloak v18+

[martincostello]

Support changes to the resource paths in Keycloak 18.0+.

Tested locally using the Docker image following these instructions.

Client configuration

.AddKeycloak(options =>
{
    options.BaseAddress = new Uri("http://localhost:8080");
    options.ClientId = "myclient";
    options.ClientSecret = string.Empty;
    options.Realm = "myrealm";
    options.Version = new Version(19, 0);
    options.AccessType = AspNet.Security.OAuth.Keycloak.KeycloakAuthenticationAccessType.Public;
})

Server configuration

Successful login

Resolves #695.