[stable] std.container.rbtree: Refactor template constraint wrt. less and use it for redBlackTree() functions too

[kinke]

PR #10792 changed the constraint to allow const-ref predicates too, using a lambda - which seems to have a very bad effect on attributes inference. Avoiding the lambda and using std.traits.lvalueOf() instead seems to work better.

Also update the template constraints of the free-standing redBlackTree() functions, which were forgotten in #10792.

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