Bug: Fix sorting Closed at and Merged At sorting options to be only available on relevant states

Following on from this suggestion: #18458 (comment 637627065) at the moment we allow Merged At and Closed at to be used on the merge request state tabs that are not relevant.

For example, we can sort by Merged At on Open, this doesn't make too much sense. So we should allow the user to only sort Merged At in Merged, and Closed At in Closed.