Core: `ignore_unavailable` shouldn't ignore closed indices

The `ignore_unavailable` request setting shouldn't ignore closed indices if a single index is specified in a search or broadcast request.

PR for #7153
I see what you mean, cause `failClosed` is used elsewhere as well... oh man this method would reuse a rewrite, I always wondered if we should just remove all these optimizations around the single index case and this `possiblyAliases` check as well.
I would consider copy pasting the check anyways because you added an `if` in the method that has to do with `forbidClosedIndices`, it should make things more readable.