Update plugins.rst examples to use pyproject.toml over setup.py

[hterik]

pyproject.toml is the standard project description moving forward. Airflow has itself adopted this recently. Examples should follow this to ease using the latest standard for plugin authors.

https://packaging.python.org/en/latest/discussions/setup-py-deprecated/#is-pyproject-toml-mandatory

[Taragolis]

• Tests / Static checks, mypy, docs / Static checks (pull_request): Rebase your branch to the current main and this should be gone
• Tests / Static checks, mypy, docs / Build documentation (--spellcheck-only) (pull_request) add toml into the https://github.com/apache/airflow/blob/main/docs/spelling_wordlist.txt