ngtcp2: Support the latest update key callback type

ngtcp2: Support the latest update key callback type

- Remove our cb_update_key in favor of ngtcp2's new
  ngtcp2_crypto_update_key_cb which does the same thing.

Several days ago the ngtcp2_update_key callback function prototype was
changed in ngtcp2/ngtcp2@42ce09c. Though it would be possible to
fix up our cb_update_key for that change they also added
ngtcp2_crypto_update_key_cb which does the same thing so we'll use that
instead.

Ref: https://github.com/ngtcp2/ngtcp2/commit/42ce09c

Closes https://github.com/curl/curl/pull/4735