Find a replacement for `oidc-client-js`

### Current Behavior:

The OIDC client library `oidc-client-js` we currently use has been unmaintained and archived for a while now (https://github.com/IdentityModel/oidc-client-js/issues/1391). We need to find a replacement for it that is still maintained.

### Proposed Behavior:

Migrate to the TypeScript-focused fork [`oidc-client-ts`](https://github.com/authts/oidc-client-ts). It is [largely compatible](https://github.com/authts/oidc-client-ts/blob/main/docs/migration.md) with `oidc-client-js` and will not require major refactoring.

Only drawback that will have user impact: It doesn't support the `implicit` flow anymore. AFAICT, this will bite users who use Google Identity, as Google doesn't support "public" OIDC clients for the authorization code flow.