Add a lower-bound pin for the `flake8-docstrings` `pip` package

[jsf9k]

🗣 Description

I noticed that flake8-docstrings is listed as an additional dependency of flake8 in the pre-commit configuration, but pinning is done.

💭 Motivation and context

@mcdonnnj agreed with me that it makes sense to at least perform some minimal pinning to ensure that a working version of flake8-docstrings is installed.

🧪 Testing

All automated tests pass. I also verified that there are no other additional dependencies in the pre-commit configuration that do no at least have a lower-bound pin.

✅ Pre-approval checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] All new and existing tests pass.