[CS] Upgrade a couple of tuple warnings to error for future lang mode #85188
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I missed upgrading the tuple label mismatch warning to an error for Swift 6 mode, let's upgrade it to an error for a future language mode. It's important we reject these cases since we're otherwise allowing subtyping to be a weaker constraint than conversion. While here let's do the same for tuple shuffles, I also took the opportunity to improve the wording of the diagnostic a bit.