Do not prefill answer in `What is the oldest version of Rust you use for any development task?`

[Kobzol]

The What is the oldest version of Rust you use for any development task? question has a pre-filled answer (the latest nightly), which could skew the results. We should probably set an empty answer as the default.

Relatedly, we could add a "I don't know" answer here.

Discussed on Reddit.

Screenshot for archiving purposes