This mirrors a similar side fix for the API server authenticator.
Don't trim the issuer URL provided by the user since OpenID Connect
mandates that this URL exactly matches the URL returned by the
issuer during discovery.

Not test updates since this is already tested by the go-oidc client
package.

See: https://openid.net/specs/openid-connect-discovery-1_0.html#ProviderConfigurationValidation