`drop_value` algorithm naming

[msimberg]

There's potential for misunderstandings with the current drop_value sender adaptor. It's currently implemented with the semantics of then([](auto&&...) {}), i.e. all value categories are accepted, but always by reference, so the values will actually be destroyed whenever an operation state decides that it's time to do so.

An alternative name for the current semantics could be ignore_value. More appropriate semantics for the drop_value name could be then([](auto...) {}).