elliptic-curve: fix hkdf version requirement and test in CI

[tarcieri]

The minimal supported version of hkdf is v0.12.1, which added a generic parameter with a default which selects the backend.

This also (mostly) re-enables a minimal-versions check in CI, although it still doesn't quite test all features.

[tarcieri]

This is now catching the hkdf version requirement error:

https://github.com/RustCrypto/traits/actions/runs/6382152698/job/17320137326?pr=1353

   Compiling elliptic-curve v0.13.5 (/home/runner/work/traits/traits/elliptic-curve)
error[E0432]: unresolved import `hkdf::hmac`
  --> src/ecdh.rs:36:12
   |
36 | use hkdf::{hmac::SimpleHmac, Hkdf};
   |            ^^^^ could not find `hmac` in `hkdf`

error[E0107]: struct takes 1 generic argument but 2 generic arguments were supplied
   --> src/ecdh.rs:192:54
    |
192 |     pub fn extract<D>(&self, salt: Option<&[u8]>) -> Hkdf<D, SimpleHmac<D>>
    |                                                      ^^^^    ------------- help: remove this generic argument
    |                                                      |
    |                                                      expected 1 generic argument
    |
note: struct defined here, with 1 generic parameter: `D`
   --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/hkdf-0.12.0/src/lib.rs:199:12
    |
199 | pub struct Hkdf<D>
    |            ^^^^ -

Note that there are some others which I'll try to address separately, namely with the bits feature (wyz) the jwk feature (serde_json).