[docs] Update Code Conventions to reflect the switch from flake8 to ruff

[jimmmmmmmmmmmy]

The Code Conventions page should be updated to replace mentions of flake8 with ruff to reflect the current linter setup

https://github.com/python/mypy/wiki/Code-Conventions

This commit from June 20, 2023, introduces ruff into the pyproject.toml file and removes flake8 from the configuration.

It looks like the pyproject.toml file's[tool.ruff] section configures ruff to use the same linting rules as flake8, just looks like a minor change is needed to the Code Conventions page.