Consider requiring attrs >= 20.1.0 for "next-gen" `attrs.mutable` and `attrs.frozen`

This issue has been migrated from [#13243](https://github.com/matrix-org/synapse/issues/13243).

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Attrs 20.1.0 introduced a "Next-gen API", see https://www.attrs.org/en/stable/names.html#tl-dr . From https://www.attrs.org/en/stable/api.html#attrs.define this basically contributes a bunch of sensible defaults (auto-detect auto_attribs; slots by default, opt-in ordering). This is slightly nicer from our POV as it makes it harder to forget to write `slots=True` etc.