Filter out indices in the caches which are larger than the validators in the state

[StefanBratanov]

PR Description

This was the logic before and got removed by mistake in #8165

final Optional<Integer> validatorIndex = validatorIndices.getCached(publicKey);
if (validatorIndex.isPresent()) {
   return validatorIndex.filter(index -> index < state.getValidators().size());
}

Fixed Issue(s)

N/A

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.