waiverdb: Fix redirect_uri for new flask-oidc versions
For details, see the changelog for flask-oidc:
https://github.com/fedora-infra/flask-oidc/blob/develop/docs/changelog.md
The entry for 2.0.0 says:
> The callback route (aka "redirect URL") is not configurable with
> `OIDC_CALLBACK_ROUTE` anymore. It is always `/authorize`,
The option `OIDC_CALLBACK_ROUTE` is available again in version 2.0.3,
but we can override the whole redirect URL with
`OIDC_OVERWRITE_REDIRECT_URI` (since 2.1.0).