Update nightly version in CONTRIBUTING.md to 2023-02-17
Summary:
D13145 bumped the nightly version, update the instructions in CONTRIBUTING.md accordingly.
Note: To uninstall the old nightly version, run rustup toolchain uninstall nightly-2022-06-29
Test Plan: Paste the new install instructions for the "abc-nightly" toolchain into a command line (should be done for future diffs anyway).
Reviewers: Fabien, #bitcoin_abc
Reviewed By: Fabien, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D13146