Improve error for trailing `/` in issuer url

### oidcc version

3.3.0

### Erlang version

27

### Elixir version

1.18.1

### Summary

Using an issuer of `https://cognito-idp.eu-central-1.amazonaws.com/…/` causes an issuer mismatch error, while the same url without trailing `/` doesn't. I'm not sure if this is expected. It either could have a better error message / be detected earlier if not be silently handled by removing the trailing `/`.

### Current behavior

See above.

### How to reproduce

See above.

### Expected behavior

See above.