Replace pre-commit/mirrors-prettier with rbubley/mirrors-prettier

[manics]

pre-commit/mirrors-prettier@v4.0.0-alpha.8 is showing inconsistent behaviour in several JupyterHub repos- pre-commit.ci is pushing unrelated changes to some PRs, which get reverted when pre-commit is run locally.

pre-commit/mirrors-prettier is no longer supported. rbubley/mirrors-prettier is a fork that is supported, and uses the latest release of prettier (v3.6.2) instead of an alpha release (v4.0.0-alpha.8).

This change has been made in a few repos:

• Pre commit fixes: switch to supported mirrors-prettier, workaround beautysh bug zero-to-jupyterhub-k8s#3705
• Switch to rbubley/mirrors-prettier, downgrade prettier to last proper release v3.6.2 jupyterhub#5092

This pre-commit bug is showing up in other repos such as
jupyterhub/mybinder.org-deploy#3363

I think we should switch to rbubley/mirrors-prettier@v3.6.2 in every repo that uses pre-commit prettier.