feat: AnswerExactMatchEvaluator input should be one answer per query instead of list of answers

[julian-risch]

We should change the inputs of the AnswerExactMatchEvaluator added in #7050 from
questions: List[str], ground_truth_answers: List[List[str]], predicted_answers: List[List[str]]
to
ground_truth_answers: List[str], predicted_answers: List[str]

This change will make all metrics in Haystack core and in integrations, statistical and model-based, consistent in the inputs they expect. For answers always List[str], for queries always List[str] and for documents (contexts) always List[List[str]]. It also simplifies the implementation of the new metrics and will allow us to move faster.

Describe alternatives you've considered
Keeping inputs as is would mean they are inconsistent with the model based metrics and the integrations of evaluation frameworks. However, the behavior would be the same as the exact match metric in Haystack 1.x and more flexible for datasets with multiple ground truth answers such as SQuAD 2.0 and multiple predicted answers like our Reader's output.

[shadeMe]

Mentioning this here so that we don't forget - We should also remove the questions input parameter as it's unused.

[masci]

Mentioning this here so that we don't forget - We should also remove the questions input parameter as it's unused.

Removed in #7466