Rails 8 needs to use credentials instead of secrets for env variables

Maybe I'm doing something wrong but I can't access `/admin` without an error. Please correct me!

When I try to access `/admin` with the correct credentials, I get the following error:

```
undefined method `secrets' for an instance of Speakerline::Application
```

I believe this is because Rails ~8~ 7.2 removed `.secrets` in favor of `.credentials`.

Maybe we could get rid of dotenv gem too