Fully support right outer joins

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LeftJoin and RightJoin are being introduced into .NET 10 (dotnet/runtime#110292); support for the new LeftJoin operator in EF is tracked by #12793. For RightJoin, we should eventually implement actual support for it (i.e. translate LINQ RightJoin to SQL RIGHT JOIN). However, until that's done, we can provide a default implementation that simply transforms RightJoin to LeftJoin.