Update to schnorrkel `0.8.0` (#3267)

* Update to schnorrkel `0.8.0`

* Increase `spec_version`

* Bump schnorrkel to 0.8.3 (#3283)

* Schnorrkel 0.8.1 (builds on bkchr upgrade branch)

* Add tests for known hard/soft derivation values

* Bump all schnorrkel versions

* Flatten for easier inspection

* 0.8.2

* 0.8.3

* Update subkey/Cargo.toml

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Update `Cargo.lock`

* 0.8.4

* Fix cargo lock file.

* Adding an error message for `NotMarkedSchnorrkel` as that was missing.
Also fixing a typo, strage -> stage.